citra: Remove GLFW, Add SDL2

FindSDL2: Do not CACHE SDL2_* variables if library is not found

EmuWindow_SDL2: Set minimal client area at initialisation time

EmuWindow_SDL2: Corrections

EmuWindow_SDL2: Fix no decorations on startup on OS X

cmake: windows_copy_files

This commit:
* Adds a new subproject, audio_core.
* Defines structures that exist in DSP shared memory.
* Hooks up various other parts of the emulator into audio core.

This sets the foundation for a later HLE DSP implementation.

Several cleanups to the buildsystem:
 - Do better factoring of common libs between platforms.
 - Add support to building on Windows.
 - Remove Qt4 support.
 - Re-sort file lists and add missing headers.