wmaker: add new ModifierKeyShortLabels option

This patch is adding a new ModifierKeyShortLabels option to the
WindowMaker file to let the user specify the modifier key labels
used in the shortcuts like those appearing in the root menu.
For example, to overwrite the default labels, a user can set
the new option value to:

ModifierKeyShortLabels = (
  "\342\207\247",
  "\342\214\203",
  "\342\214\245",
  "\342\207\255",
  "\342\207\263",
  "\342\214\230",
  "\342\207\252",
  "\342\227\206",
  "\342\214\245"
);

Which is using the same symbols as defined in macos.
For example, instead of printing M4+, "\342\214\230"
will print the ⌘ (Command) symbol.

src/WindowMaker.h

@@ -393,6 +393,7 @@ extern struct WPreferences {
 	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 *modifier_short_labels[9];    /* Short names of the modifiers */
 
 	unsigned int supports_tiff;        /* Use tiff files */