Open Submenu on Shortcut

[Zren]

As mentioned in Issue #1.

• [ ] Open Submenu on Shortcut.
  • SublimeText has mnemonics, and opens app submenus at the top left of the window (relative (0,0)). So I'll probably need to adjust the position during an aboutToOpen signal or something. Or update it's position during updateButtonGeometry.
  • The shortcut opens the LIM submenu instead of the Global Menu in Sublime Text, which is a good sign.
  • Decorations don't recieve KeyEvents afaik. So it's not possible to detect Alt pressed. I don't think it's possible to detect a menu shortcut either. Gtk menus open (in the wrong spot), but Qt menus don't at all.
    • https://github.com/KDE/kwin/blob/6abd23ed0220b14a4a05fa280869ba3c953565e3/input.cpp#L1076
• [ ] Display mnemonics when holding Alt.

This requires changes upstream in KWin to implement.