xwm: use override_redirect for determining or not top-level windows
xwm: use override_redirect for determining or not top-level windows

"top-level window is a window whose override-redirect attribute is False",
ICCCM 4.1.1

Signed-off-by: Tiago Vignatti <f080760b2054d506aacc8dee97f8804c19fbc3aa@intel.com>

eyolfson/weston,krezovic/weston,eyolfson/weston,xorgy/weston,Tarnyko/weston-xdg_surface_present,giucam/weston,Fantu/compositor-spice,Tarnyko/weston-xdg_surface_present,Gnurou/weston,udoprog/weston,kwm81/weston,kwm81/weston,mchalupa/weston,sir-murray/weston,krezovic/weston,mchalupa/weston,Gnurou/weston,sir-murray/weston,Fantu/compositor-spice,Fantu/compositor-spice,jonnylamb/weston,udoprog/weston,xorgy/weston,giucam/weston,jonnylamb/weston