OS X native backend no longer use files from src/sdl directly

Cocoa backend is compiling but failed to link
Some files are referenced using symbolic links
Apple's HID Utilities were moved to own directory
Get rid of SDL in more parts of native OS X backend
Video part of native OS X backend as a copy-paste of SDL one

Dependency from SDL still exists, pending major refactoring
Rearranged files to avoid usage of symbolic links
Merge branch 'master' of https://github.com/rheit/zdoom into z_osx_pure
Added missing #include
Removed unnecessary SDL #include's
No more SDL dependency in native OS X backend
Fixed path to SDL specific Obj-C++ file
Native OS X backend no longer requires SDL
Put all non-Windows source code into posix directory and its subdirectories
Fixed build of SDL output plug-in
Cleanup video part of native OS X backend, phase 1
Cleanup video part of native OS X backend, phase 2
Cleanup video part of native OS X backend, phase 3
Cleanup video part of native OS X backend, phase 4
Merge branch 'z_osx_pure' into cocoa_no_sdl