Linux: added SDL2 backend as replacement for GLFW.

Initially planned as alternative, made replacement after discussion with
minggo.

Summary:
GLFW v.3 released in June. New version breaks API without real need:
http://cocos2d-x.org/boards/6/topics/31277
Meanwhile SDL2 is very stable. SDL2 used in Steam Runtime and will be
part of Ubuntu SDK, it's supported by other distributions.

Completed:
- OpenGL initialization and window handling with SDL
- Keyboard, IME emulation, mouse and touches support with SDL
- Window title now contains app name on Linux
- Backspace correctly handled with new backend

Note:
- SDL on X11 have no proper multitouch, but backend handles multitouch
events to support future Linux graphics.
Linux: added SDL2 to travis scripts and install-deps-linux.sh
Merge pull request #3094 from sergey-shambir/linux_sdl

closed 2458: Linux: added SDL backend as alternative to GLFW.