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

include/config/keybind.h

@@ -20,6 +20,7 @@ struct keybind {
 	int keycodes_layout;
 	struct wl_list actions;  /* struct action.link */
 	struct wl_list link;     /* struct rcxml.keybinds */
+	bool on_release;
 };
 
 /**