The root window contains all the monitors attached to a Mac.  The
coordinate transformation now both translates the x and y coordinate,
translating it from the Cocoa monitor coordinate space to the GDK
coordinate space.  How monitors are laid out in the root window differs
between Cocoa and GDK, which is why it is important to translate based
on the root window to get multi monitor setups to work properly.

We have replaced the old y coordinate transformation function with
new functions that translate both the x and y coordinate.

When creating new toplevels, we have to determine the Cocoa screen on
which the toplevel should appear and translate the coordinates according
to that screen.

This change also fixes event handling in case there is a monitor left
of the screen containing the menu bar.  In such a case all coordinates
on the left monitor are negative.  Event handling broke, because of
_gdk_quartz_window_find_child() checking bounds.  Now that coordinates
are always properly translated to GDK coordinate space, in which negative
coordinates do never occur, the checks here will work properly.