westoy: Use subsurfaces for tooltips instead of transient windows

Transient windows, at least not as they are today, don't exist in
xdg_shell. Subsurfaces allow for specially placed surfaces relative
to a window, so use these instead.
westoy: Remove fullscreen methods

These aren't supported under xdg_shell, at least not right now.
If xdg_shell ever gets support for them, we can revert this commit
and adapt it.
westoy: Remove MAXIMIZED and FULLSCREEN as separate window types

xdg_shell changes this around so that they are flags on the remote
object itself, not separate surface types. Move to a system where
we calculate the state from the flags ourselves and set the appropriate
wl_shell_surface type.

When we port to xdg_shell, we'll drop these flags and simply sync
on the client.
westoy: Schedule a full resize when we add a subsurface

If a client adds a subsurface, we need to make sure it's allocated
properly, so queue a resize and redraw on the parent window.
westoy: Port the toy toolkit over to xdg-shell
toytoolkit: Add check for window->custom.
xdg-shell: Send next fullscreen maximized states on configure.

The current states will only take effect on next commit, which comes
after the configure was processed by the client. Thus, the client must
receive the next states, not the current ones.