On X11 we receive enter notify and motion notify events for a window
regardless of its focus state.  On Mac OS X this is not the case.  This
commit improves the semantics to overcome this difference.  It improves
on my earlier patch that sent a motion notify event when a window became
main.

Instead of sending a motion notify when a window becomes main, we now
send one when a window becomes key, which comes closest to a window
getting focus in X11.  This motion notify is needed because Mac OS X does
not send motion events when an application is inactive (none of its
windows have focus), these events are sent in X11.  This dummy motion
notify event (with current coordinates of the mouse cursor) allows an
application to get its prelight and other state right when it gets focus
and thus user attention.

Another change is to send an enter notify event when updating the
tracking rectangle of a GdkQuartView and the mouse cursor is currently in
this rectangle.  This rectangle is at least updated on window creation.
This enter notify event is important for the case where a new window
appears right below the mouse cursor.  The window has to receive an enter
notify event for the subsequent events to be processed correctly.  Mac
OS X does not send one in this case, so we generate it ourselves.

Both of these synthesized events have to go through
_gdk_windowing_got_event() for updating statekeeping, etc.
append_event() has a boolean flag now to make this convenient.