macos: fix window activation during shadow click-through

If we are clicking through the shadow of a window, we need to take special
care to not raise the old window on mouseUp. This is normally done by the
display server for us, so we need to use the proper API that is public to
handle this (rather than CGSSetWindowTags()). Doing so requires us to
dispatch the event to the NSView and then cancel the activcation from
the mouseDown: event there.