Fixed bug 2631 - Mac: minor code cleanup

Alex Szpakowski

Some minor changes to the Mac-specific backend code:

- Fixed up some code style issues (mostly brace style inconsistencies).

- Fixed a compiler warning in SDL_cocoaevents.m.

- Removed some useless code now that the 10.7 SDK is required to build SDL.

- Removed Gestalt(gestaltSystemVersion, ...) call and switched to NSAppKitVersionNumber for version checking code. Using Gestalt with gestaltSystemVersion will give 0x1090 in Mac OS 10.10+, and the whole Gestalt function was deprecated in Mac OS 10.8.