added: UnloadSkin builtin function, and allow...

added: UnloadSkin builtin function, and allow ApplicationMessenger::ExecBuiltIn to wait on the builtin before returning

(cherry picked from commit a249081ab2e3d4b2b5348607df6a3513522bc40f)

git-svn-id: https://xbmc.svn.sourceforge.net/svnroot/xbmc/branches/Dharma@35632 568bbfeb-2a22-0410-94d2-cc84cf5bfa90