view: Make view implementations explicitly change view state

This way, the framebuffer plane can keep a constant size (even if an
oversized buffer is attached). Additionally this enables some tricks for
ensuring smooth move+resize (common operation for tiling window
managers).
compositor: Use assert to ensure view is created by compositor
Minor documentation changes
xkb: Don't use unnecessary code-block
Style, remove extraneous debug statements.
xkb: Use WARNING