Add initial view focusing code

[sdilts]

Add the required functions to focus a view in the backend and use it.

• Delay notifying the frontend of new views until the first commit is made; this allows us to immediately call events that require the xdg_toplevel to be configured.
• Guess on what we need to do to unfocs a view; it seems to work.

Also initialized the seat correctly by letting it know about the keyboard group that we are using.