Display modifiers in window menu

The window menu displays the shortcut key for operations with a
shortcut, but does not display the modifiers. This reduces the utility
of the display as it's hard to know whether "h" means "Mod4+h" or
"Ctrl+Mod4+h" or something else.

This patch prefixes those shortcut displays with the modifier names,
e.g. Ctrl+Mod4+h, and adds a preference ModifierKeyLabels to allow
overriding this, e.g. to "⌃◇h". It doesn't add this preference to
WPrefs, if someone else wants to do that feel free.

Signed-off-by: Brad Jorsch <anomie@users.sourceforge.net>

src/funcs.h

@@ -65,6 +65,8 @@ void OpenWorkspaceMenu(WScreen *scr, int x, int y);
 
 void CloseWindowMenu(WScreen *scr);
 
+void DestroyWindowMenu(WScreen *scr);
+
 WMagicNumber wAddDeathHandler(pid_t pid, WDeathHandler *callback, void *cdata);
 
 void wColormapInstallForWindow(WScreen *scr, WWindow *wwin);