This adds a gesture handle mode to the navigation panel, which can be
enabled during gesture-only mode. This reserves space for the system
gesture to be able to be used, allowing us to extend the height in KWin
of the gesture recognition area (which is currently far too short for devices such as Pixel 3a).
This also allows for navigation with a mouse; clicking on the handle
triggers the task switcher, holding it triggers the "home" action.