Linux: added SDL backend as alternative to GLFW.

Summary:
GLFW v.3 released in June. New version breaks API without real need:
http://cocos2d-x.org/boards/6/topics/31277
Meanwhile SDL is very stable. SDL2 used in Steam Runtime and will be
part of Ubuntu SDK, it's supported by other distributions.

Completed:
- Window title now contains app name
- OpenGL initialization and window handling with SDL
- Keyboard, text edit, mouse and touches support with SDL

Notes:
- SDL on X11 have no proper multitouch, but backend handles multitouch
events to support future Linux graphics.