Add onRelease option to <keybind>

Add onRelease option to <keybind>

...to make keybind actions fire on the release event rather then when the
key is first pressed. This is useful for binding actions to modifier keys
only. The most likely use-case for this is the binding of a Super key to a
menu, for example:

    <keybind key="Super_L" onRelease="yes">
      <action name="Execute" command="rofi -show drun"/>
    </keybind>

If another keybind is issued between the press and release, the on-release
keybind is cancelled.

Co-authored-by: @johanmalm