wayland: Add support for xdg_toplevel.bounds

wayland: Add support for xdg_toplevel.bounds

The GdkToplevelSize struct already has the concept of "bounds", which
means the largest size a window should reasonably have. It's practically
the equivalent of the monitor the window is intended to be mapped on,
with the "struts" (e.g. panels) cut out. It's used by GTK to use this
information to calculate a default window size that is "lagom" (swedish;
not too large, not too small).