NS/Appkit: frame visibility and application activated

[David Reitter]

After doing (make_frame_invisible nil t) on the only visible frame, and then 
switching the application away and back again to Emacs,

- the NS port makes the frame visible, while
- the Mac/Appkit port doesn't make the frame visible.

The latter is the desired behavior.

In the NS port, the frame seems to be made visible (ordered in front) by the 
system, after the application receives the applicationWillBecomeActive: 
message, and despite my best efforts, I haven't figured out why that is, or why 
the Appkit port gets it right.  There is a "needsOrderFrontOnUnhide" flag for 
each frame, but that seems to apply only during Unhide, not in 
ApplicationActivated situations.