We can set the maximized and state_changed flags in set_maximized(),
which is shared between shell_surface_set_maximized() and
xdg_surface_set_maximized().
Since internally there's no more SHELL_SURFACE_FULLSCREEN and
SHELL_SURFACE_MAXIMIZED, the surface must be set to
SHELL_SURFACE_TOPLEVEL on the respective functions.
This fixes the bug when clients start already in fullscreen mode. In
that case, they aren't set first to toplevel, and then change to
fullscreen. They are set as fullscreen directly, not receiving the
SHELL_SURFACE_TOPLEVEL type on the set_fullscreen function.
