Add the initial SDL_Window wrapper code.

With this wrapper in place, Wesnoth shows a main window, all black and a
lot of errors on the console.
Implement rendering with SDL 2.

The game now starts and can be played. There are a lot of visual
glitches, but visual it is playable.

The text input is completely broken, due to the changes in the
SDL 2 API and the band-aid compile fixes.
Move window caption initialisation code.

The code is moved from game_controller::init_language to
game_controller::init_video. This makes porting the code to SDL 2
easier. In SDL 2 the caption must be set after the main window exists.
It could be set in the constructor of the Window, but that can be done
once we drop SDL 1.2 support.
Show the window caption with SDL 2.
Show the window icon with SDL 2.
Let CVideo::window_state use main_window.
Rename main_window to window.

With the last commit the duplicate variable name window is gone, so
rename the main_window to the preferred name window.
Update the Doxygen comments.
Merge branch 'SDL_Window'

Add a wrapper for the SDL_Window. This merge makes Wesnoth 'playable'
with SDL2. It still has a lot of glitches and bugs to be solved.