Show mnemonics in the topbar when pressing Super

[hedning]

When pressing Super we should show hints in at least the topbar (sems like that's what the gnome people are calling it). So eg. an M beside the calendar popdown.

[olejorgenb]

Note: To bind modifier-only keybindings simply use the keysym as if it was a regular key. (eg. Super_L not <Super>)

[olejorgenb]

It's possible to use Gdk.Keymap to detect both keydown and keyup for modifers.

    const keymap = Gdk.Keymap.get_default();
    keymap.connect('state_changed', () => {
      keymap.get_modifier_state() & Gdk.ModifierType.CONTROL_MASK
    });

Ref: https://github.com/sneetsher/keyboard_modifiers_status

Edit: WIP branch for general modifer tracking: https://github.com/paperwm/PaperWM/tree/modifier-tracker