Add a SDL2 compatibility wrapper.

This wrapper takes care of the renaming of some types done in SDL2. This
allows better compatibility between SDL 1.2 and 2.0 code.
Rename a function parameter unused -> alpha.

The new name is more appropriate.
Add a fix for the renamed field in SDL2.

The unused field in SDL_Color has been renamed to a.
Port sdl_keysym_from_name for SDL2 usage.

Use the SDL2 function SDL_GetKeyFromName.
Add a wrapper for SDL_SetAlpha when using SDL2.

The code is only compile-tested. Since there are more compilation issues
the code has not been tested at run-time.
Fix a several event handling loops.

Several events which were normal events in SDL1.2 have been moved as
subevents of the SDL_WINDOWEVENT in SDL2.0.
Port SDL_SetPalette for SDL2 usage.
Port SDL_GetKeyState for SDL2 usage.
Fix linker issues with SDL2.

The compatiblity file sdl/alpha.cpp is now linked properly in CMake,
Scons has not been tested yet.
Disable UNICODE setting for SDL2.

The calls for setting UNICODE are no longer used in SDL2.
Remove the SDL_EnableKeyRepeat call for SDL2.

The function is no longer available.
Change a type to be SDL2 compatible.
Avoid using the unicode field in SDL2.

The unicode field in the SDL_keysym no longer exists in SDL2. Use the
scancode field instead. This needs to be evaluated later.
Add a work-around for the META rename in SDL2.

Several foo_META constants have been renamed to foo_GUI in SDL2.
Add a SDL_FULLSCREEN work-around for SDL2.
Add a work-around for SDL_EVENTMASK in SDL2.
Add a work-around for SDL_GetAppState in SDL2.

The code only compiles, but never return a sensible value. This should
be done later.
Disable some code for SDL2.

The code doesn't compile for SDL2 and it seems a work-around for an
issue in SDL, so maybe it is no longer required with SDL2. This needs to
be tested later.
Refactor show_video_mode_dialog.

Separated the dialog code from the resolution gathering code. This makes
it easier to port the code to SDL2.

This also changes the sorting of the resolutions, first width then
height instead of based on the area. Before a part of the list looked
like:
1280 x 720
1152 x 864
1280 x 960
1440 x 900
1282 x 1024
These now look like:
1152 x 864
1280 x 720
1280 x 960
1280 x 1024
1440 x 900
The new sorting makes more sense to me.
Fix another SDL_ACTIVEEVENT usage for SDL2.

Not sure whether this code is a work-around for somthing SDL1.2 specific
or is also required for SDL2.