lomiri / lomiri-system-settings / lomiri-schemas: Make width of side stage & indicator menu configurable

When switching Lomiri to staged mode (System/Session indicator -> disable "Computer Mode"), you can three-finger-swipe apps to the so-called side stage (on the right).

On very high resolution displays with a low GRID_UNIX_PX value, the width of the side stage could be wider. The width of the side stage matches the width of the indicator menu area.

The width of both areas (indicator menu, side stage) should be made configurable via lomiri-system-settings (plugins/launcher/).