Initial app switcher

When pressing the home screen button toplevels form xdg_surfaces can be selected. This will do until we switch to the wlroots protocoll that is currently in the works). Most of the current code can then be reused.

This needs https://source.puri.sm/Librem5/wlroots/merge_requests/16 to go in first.

Marking as WiP since this needs a newer wlroots first.