Add a system of clipboard identifiers.

This lays some groundwork for making PuTTY's cut and paste handling
more flexible in the area of which clipboard(s) it reads and writes,
if more than one is available on the system.

I've introduced a system of list macros which define an enumeration of
integer clipboard ids, some defined centrally in putty.h (at present
just a CLIP_NULL which never has any text in it, because that seems
like the sort of thing that will come in useful for configuring a
given copy or paste UI action to be ignored) and some defined per
platform. All the front end functions that copy and paste take a
clipboard id, and the Terminal structure is now configured at startup
to tell it which clipboard id it should paste from on a mouse click,
and which it should copy from on a selection.

However, I haven't actually added _real_ support for multiple X11
clipboards, in that the Unix front end supports a single CLIP_SYSTEM
regardless of whether it's in OS X or GTK mode. So this is currently a
NFC refactoring which does nothing but prepare the way for real
changes to come.

windows/windlg.c

@@ -140,7 +140,7 @@ static INT_PTR CALLBACK LogProc(HWND hwnd, UINT msg,
 			    memcpy(p, sel_nl, sizeof(sel_nl));
 			    p += sizeof(sel_nl);
 			}
-			write_aclip(NULL, clipdata, size, TRUE);
+			write_aclip(NULL, CLIP_SYSTEM, clipdata, size, TRUE);
 			sfree(clipdata);
 		    }
 		    sfree(selitems);

windows/window.c

@@ -637,6 +637,8 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmdline, int show)
      * timer_change_notify() which will expect hwnd to exist.)
      */
     term = term_init(conf, &ucsdata, NULL);
+    term->mouse_select_clipboard = CLIP_SYSTEM;
+    term->mouse_paste_clipboard = CLIP_SYSTEM;
     logctx = log_init(NULL, conf);
     term_provide_logctx(term, logctx);
     term_size(term, conf_get_int(conf, CONF_height),
@@ -2328,10 +2330,10 @@ static LRESULT CALLBACK WndProc(HWND hwnd, UINT message,
 	    }
 	    break;
 	  case IDM_COPYALL:
-	    term_copyall(term);
+	    term_copyall(term, CLIP_SYSTEM);
 	    break;
 	  case IDM_PASTE:
-	    request_paste(NULL);
+	    term_request_paste(term, CLIP_SYSTEM);
 	    break;
 	  case IDM_CLRSB:
 	    term_clrsb(term);
@@ -2563,7 +2565,7 @@ static LRESULT CALLBACK WndProc(HWND hwnd, UINT message,
 	break;
       case WM_DESTROYCLIPBOARD:
 	if (!ignore_clip)
-	    term_deselect(term);
+	    term_lost_clipboard_ownership(term, CLIP_SYSTEM);
 	ignore_clip = FALSE;
 	return 0;
       case WM_PAINT:
@@ -4146,7 +4148,7 @@ static int TranslateKey(UINT message, WPARAM wParam, LPARAM lParam,
 	    return 0;
 	}
 	if (wParam == VK_INSERT && shift_state == 1) {
-	    request_paste(NULL);
+	    term_request_paste(term, CLIP_SYSTEM);
 	    return 0;
 	}
 	if (left_alt && wParam == VK_F4 && conf_get_int(conf, CONF_alt_f4)) {
@@ -4861,11 +4863,15 @@ void palette_reset(void *frontend)
     }
 }
 
-void write_aclip(void *frontend, char *data, int len, int must_deselect)
+void write_aclip(void *frontend, int clipboard,
+                 char *data, int len, int must_deselect)
 {
     HGLOBAL clipdata;
     void *lock;
 
+    if (clipboard != CLIP_SYSTEM)
+        return;
+
     clipdata = GlobalAlloc(GMEM_DDESHARE | GMEM_MOVEABLE, len + 1);
     if (!clipdata)
 	return;
@@ -4905,13 +4911,17 @@ int cmpCOLORREF(void *va, void *vb)
 /*
  * Note: unlike write_aclip() this will not append a nul.
  */
-void write_clip(void *frontend, wchar_t *data, int *attr, truecolour *truecolour,
-                int len, int must_deselect)
+void write_clip(void *frontend, int clipboard,
+                wchar_t *data, int *attr, truecolour *truecolour, int len,
+                int must_deselect)
 {
     HGLOBAL clipdata, clipdata2, clipdata3;
     int len2;
     void *lock, *lock2, *lock3;
 
+    if (clipboard != CLIP_SYSTEM)
+        return;
+
     len2 = WideCharToMultiByte(CP_ACP, 0, data, len, 0, 0, NULL, NULL);
 
     clipdata = GlobalAlloc(GMEM_DDESHARE | GMEM_MOVEABLE,
@@ -5372,8 +5382,10 @@ static void process_clipdata(HGLOBAL clipdata, int unicode)
     sfree(clipboard_contents);
 }
 
-void request_paste(void *frontend)
+void frontend_request_paste(void *frontend, int clipboard)
 {
+    assert(clipboard == CLIP_SYSTEM);
+
     /*
      * I always thought pasting was synchronous in Windows; the
      * clipboard access functions certainly _look_ synchronous,

windows/winstuff.h

@@ -55,6 +55,10 @@ struct FontSpec *fontspec_new(const char *name,
 #define PLATFORM_IS_UTF16 /* enable UTF-16 processing when exchanging
 			   * wchar_t strings with environment */
 
+#define PLATFORM_CLIPBOARDS(X)                      \
+    X(CLIP_SYSTEM, "system clipboard")              \
+    /* end of list */
+
 /*
  * Where we can, we use GetWindowLongPtr and friends because they're
  * more useful on 64-bit platforms, but they're a relatively recent
@@ -247,7 +251,7 @@ GLOBAL void *logctx;
  * which takes the data string in the system code page instead of
  * Unicode.
  */
-void write_aclip(void *frontend, char *, int, int);
+void write_aclip(void *frontend, int clipboard, char *, int, int);
 
 #define WM_NETEVENT  (WM_APP + 5)