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>