Allow to select OSK layouts

Configuring keyboards in Settings is great but the user has no indication if there's a corresponding OSK layout. By getting the list of Layouts from the OSK we can limit the selection to actually available layouts (implemented for phosh-osk-stub). We preserve any additional input settings as the user might want to use them in docked mode.

To work this needs guidog/phosh-osk-stub!149 (merged) but we'll fall back gracefully if the need information isn't found.