And also change the existing WM_KILLFOCUS handler to return 0 instead
of 'break' (which later calls DefWindowProcW), as MSDN says we should
do for WM_{KILL,SET}FOCUS.
It seems that the 'focused' property is now supported by all main VOs:
x11, macOS, wayland, Windows.
TCT/sixel/caca probably don't support it, and unknown with SDL.
Fixes#8868