winemac: Move the window to the front of the z-order in SetFocus if it's the foreground window and not already in the front.

Ideally, user32 would adjust its z-order for window activation as happens on
Windows.  Then, the Mac driver would just reflect such changes in the Cocoa
windows.  But user32 doesn't do that.  SetActiveWindow() and SetForegroundWindow()
just adjust focus and status.

This is an attempt to achieve a similar result.
Signed-off-by: Ken Thomases <ken@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>