Send xterm 216+ modifiers in small-keypad key escape sequences.

In the 'xterm 216+' function key mode, a function key pressed with a
combination of Shift, Ctrl and Alt has its usual sequence like
ESC[n~ (for some integer n) turned into ESC[n;m~ where m-1 is a 3-bit
bitmap of currently pressed modifier keys.

This mode now also applies to the keys on the small keypad above the
arrow keys (Ins, Home, PgUp etc). If xterm 216+ mode is selected,
those keys are modified in the same way as the function keys.

As with the function keys, this doesn't guarantee that PuTTY will
_receive_ any particular shifted key of this kind, and not repurpose
it. Just as Alt+F4 still closes the window (at least on Windows)
rather than sending a modified F4 sequence, Shift+Ins will still
perform a paste action rather than sending a modified Ins sequence,
Shift-PgUp will still scroll the scrollback, etc. But the keys not
already used by PuTTY for other purposes should now have their
modern-xterm behaviour in modern-xterm mode.

Thanks to H.Merijn Brand for developing and testing a version of this
patch.

windows/window.c

@@ -4627,7 +4627,11 @@ static int TranslateKey(UINT message, WPARAM wParam, LPARAM lParam,
             if (shift_state & 2)
                 break;
 
-            p += format_small_keypad_key((char *)p, term, sk_key);
+            p += format_small_keypad_key((char *)p, term, sk_key,
+                                     shift_state & 1, shift_state & 2,
+                                     left_alt, &consumed_alt);
+            if (consumed_alt)
+                left_alt = false; /* supersedes the usual prefixing of Esc */
             return p - output;
 
             char xkey;