slplugin.cpp uses deprecated FrontWindow() calls

The FrontWindow API used in slplugin.cpp has been deprecated since around MacOS 8.5, replacing calls to FrontWindow() with GetFrontWindowOfClass(kAllWindowClasses, true) will keep the 10.6 SDK happy in future.

Testing

Affects Mac only. The current viewer behaviour on playing youtube videos etc. full-screen should remain unchanged.

1. Play a youtube.com video on a prim.
2. Click the fullscreen button or double-click the movie to play it fullscreen.
   • Expected: The movie should play fullscreen in front of all other windows.
3. Click the button in the bottom right to exit full screen mode, or hit escape.
   • Expected: The fullscreen movie should disappear and focus should return to the Second Life window.