Use 1px border to separate OSK from the apps

This helps a lot to distinguish keyboard from app in HighContrast mode:

Before:

After:

I refrained from adding the border to adwaita-dark since visual separation is good there but am happy to add it there as well if needed.

This also adds a GtkBox between the window and the drawing area which allows for for gtk-css out of the box and will make adding more widgets (for e.g. text prediction) easier.

Ideally we'd create a EekWindow that derives from LayerSurface and then uses a ui file to reduce hand crafted GTK but I didn't want to do that without checking first and it can as easily be done afterwards.

Marking as draft because I want to test this a couple of days on device.