Add `Menu` command label option to show `SendMessage` keyboard shortcuts

[morgant]

When using the built-in Menu command block to specify menu items/labels (see mlvwm(1) manual), one can specify an Action SendMessage and pass in keyboard shortcuts (see ShortCut command in mlvwm(1)). While this will send the key input to the X11 application, the menu items don't show what the associated keyboard shortcut is. It would help users understand what keyboard shortcuts they can use if Menu labels could optionally show the keyboard shortcut (like MacOS does).