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/WindowMaker.h

@@ -370,6 +370,7 @@ typedef struct WPreferences {
     signed char menu_style;	       /* menu decoration style */
     signed char workspace_name_display_position;
     unsigned int modifier_mask;	       /* mask to use as kbd modifier */
+    char *modifier_labels[7];          /* Names of the modifiers */
 
 
     char ws_advance;                   /* Create new workspace and advance */