Bug 1178 - No enter / leave events on actors when pointer leaves the
stage window
This patch causes the Win32 backend to emit CLUTTER_LEAVE events when
a WM_MOUSELEAVE event is received in the same way that f505536 does
for the X11 backend.
Windows will only send WM_MOUSELEAVE events if they are previously
requested using TrackMouseEvent so this needs to be called whenever
the mouse enters the window. There is no WM_MOUSELEAVE event but we
can detect when the mouse enters because we get a WM_MOUSEMOVE event.
1fcddb3b10