Make picking a layout easier and add some more

One can use

POS_TEST_LAYOUT=se POS_DEBUG=force-show _build/src/phosh-osk-stub --replace

to just bring up another layout for testing (as a next step we want to be able to load a json file directly). While at that add some more layouts from gnome-shell (ideally we'd propose a common repo at some point but so far there's would be little to gain for the gnome-shell devs from that).