Add SDL 2.0 compatibility.

In SDL 2.0 the SDL_GetKeyState used scancodes where SDL 1.2 uses
keycodes. This adds a wrapper to make the code compatible with both
versions.

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56457 75b3775d-3801-0410-9e12-e2e43121521d
Add SDL 2.0 compatibility.

Transform the direct calls to SDL_GetKeyState to use the CKey class.
This activates the compatiblity wrapper of the last commit.

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56458 75b3775d-3801-0410-9e12-e2e43121521d