Move input-method to separate class

This decrufts phosh-osk-stub and allows us to move input-surface into the library so we can document properties, etc. It also gets rid of the odd calls between input-surface and osk-stub as state changes are now signaled and relevant state is a property so that it's double buffered correct. We add pending-changed signal so the debug surface can update it's state. This becomes even more useful with !8 (merged) (of which I have a rebased version on top of this). Also moving the debug surface into a widget becomes easier now.