GLMixer Code

Fixed compilation for libavcodec version < 53

Cleanup compilation:
- added DEFINE for shared memory (USE_SHAREDMEMORY from cmake)
- fix ffmpeg / libav compilation warnings
- fix for some translation compilation warnings

Compatibility FreeFrameGL 1.5

Cosmetic and FreeFrameGL compatibility changes.

Implementation of FFGL Source saving and loading from XML configuration.
Improved dialog for FreeframeGL source creation
(+ same improvements applied to Algorithm Source creation dialog)

Improved hidden keyboard actions assignment (CTRL PgUp and CTRL PgDown
for session switcher) and preference dialog.

Uniformization of file dialogs (& proposing file name for saving)
Cleanup keyboard shortcuts : the arrows should be left for moving
sources in the canvas.

Implementation of action 'Capture and copy current frame to clipboard'

Fixed compilation warnings

Use media file dialog application wide & some QFileDialog code cleanup