Adds SDL force feedback support (Thanks, taylor!)

git-svn-id: svn://svn.icculus.org/fs2open/branches/antipodes@10096 387891d4-d844-0410-90c0-e4c51a9137d3
Removes redundant code


git-svn-id: svn://svn.icculus.org/fs2open/branches/antipodes@10097 387891d4-d844-0410-90c0-e4c51a9137d3
Adds an SCP_Warning function that acts as frontend to SDL_ShowSimpleMessagebox


git-svn-id: svn://svn.icculus.org/fs2open/branches/antipodes@10098 387891d4-d844-0410-90c0-e4c51a9137d3