Tidy IInputHandler.
Split text input into its own event.
Tidy renderer dependencies.
Add SDL2 renderer based on SDL2# wrapper. Closes #3315.

SDL2# binary was built from commit 70af91f84493a924a3c1da46ae6209bb5f7222e3.
Merge pull request #4031 from pchote/sdl2

Add a SDL2-GL renderer.