Cleaned up SDL gui code (no functional changes - SDL2 support will appear in
a separate file)


git-svn-id: svn://svn.code.sf.net/p/bochs/code/trunk@12385 6c8e4198-a580-4521-a02f-c9ae86df1db9
Added initial SDL2 gui code, use --with-sdl2 to enable it.
TODO list:
- fullscreen mode, keyboard mapping, mouse wheel support
- take advantage of new SDL2 features
- Windows port not yet tested


git-svn-id: svn://svn.code.sf.net/p/bochs/code/trunk@12386 6c8e4198-a580-4521-a02f-c9ae86df1db9