Make room for an SDL 2 backend
CMakeLists.txt: Don't print Qt version on each run
FindGLM.cmake: Don't print include dir on each run
Add FindSDL2.cmake module
Add initial SDL 2 backend

Remaining issues:
- bith depth is not supported, will always use desktop depth
- still only supports a single window, like with SDL 1
- multi-monitor setups not supported yet, will pick the first screen
Remove Input::{,un}acquireDevices()

They weren't used anymore with SDL.
Pass window on input backend creation
Remember window pointer in SDL2InputBackend
SDL2Window: Follow code style for member names
Remember window pointer in SDL1InputBackend
SDL1Window: Follow code style for member names
Enable the SDL 2 backend by default
Pass "Automatic" resolution directly to the window

Don't select the highest resolution in ArxGame, the window is able to
handle the selection better anyway.
Force SDL2 to restore FULLSCREEN_DESKTOP windows
Also "Automatic" resolution on window init
SDL2: Fix window size when exiting fullscreen
Remove Window::getCursorPosition()

Wasn't used and belongs more into the input backend anyway.
Remove InputBackend::getKeyAsText()

Implementation was not backend-specific.
SDL2InputBackend: Remove redundant NULL check
SDL*InputBackend: Don't enable joystick events

We don't use them anyway!