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.

unix/gtkapp.c

@@ -147,7 +147,7 @@ static void paste_cb(GSimpleAction *action,
                      GVariant      *parameter,
                      gpointer       user_data)
 {
-    request_paste(user_data);
+    paste_menu_action(user_data);
 }
 
 static const GActionEntry win_actions[] = {