enum CPUState: rename CPU_* to STATE_*
CoreParameter: add enum CPUBackend
Don't install segfault handler in interpreter mode
AVIDump: cleanup
AVIDump: update ffmpeg/libav API usage

libav 10 was released on May 10th, 2014 and it drops support for some
long-deprecated stuff like avcodec_encode_video().
CMake: remove unused/duplicate definitions
CMake: mention ffmpeg in libav messages

We support both.
CMake: update min. libav/ffmpeg library versions
AVIDump: rename frame variables
VideoCommon: Get rid of an snprintf call in VideoConfig.cpp
Introduce the revised coding style.
Revert "Don't add segfault handler in interpreter mode"
Make the emulation stop asynchronous to prevent deadlocks.

This may expose bugs which relied on the Main Thread to be suspended in the stopping state.
Frame: OnClose now handles asynchronous behaviour of DoStop().
Core: Return CORE_STOPPING even if hardware is already uninitialized.
FrameTools: Update GUI and disable Play button when in stopping state.
Check whether the core is running instead of checking if it is unitialized.

This properly handles the stopping state and more acurately represents the intended check.