[osx] Enable motion event dispatch

By default NSWindow does not listen to mousemoved events and hence the
default behaviour for Actors using the "motion-event" signal  differs
from backend to backend.

Using setAcceptsMouseMovedEvents seems to fix it; unfortunately, I
cannot verify it, but since nobody is currently working on the Quartz
backend I guess it cannot get more broken than how currently is.

Thanks to: Michael <michael@f3k.org>

Fixes bug:

  http://bugzilla.openedhand.com/show_bug.cgi?id=1687

clutter/osx/clutter-stage-osx.c

@@ -54,6 +54,7 @@ clutter_stage_osx_state_update (ClutterStageOSX   *self,
     {
       [self setDelegate: self];
       [self useOptimizedDrawing: YES];
+      [self setAcceptsMouseMovedEvents:YES];
       [self setContentView: aView];
       [self setTitle:[NSString stringWithUTF8String: aTitle ? aTitle : ""]];
       self->stage_osx = aStage;