Add SDL 2.x bindings and drop SDL 1.x
The SDL 1.x bindings are included in 0.48 LTS and therefore still available for some time.
As replacement the SDL 2.0 bindings could be included from https://gitlab.gnome.org/GNOME/vala-extra-vapis or https://github.com/sdl2-vapi/sdl2-vapi into the main tree.
Edited by Rico Tzschichholz