GLMixer Code

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

Improved management of QFileDialogs for opening (and saving) files.

Made preview of source available in control dock widget for non playable
sources too.

Implementation of REMOVE feature for FreeFrame plugins : had to (switch
back) to give reference pointer to the FFGLPluginSourceStack (instead of
a copy) to the FFGLPluginBrowser.
Resolved the issue of vertical mirror with freeFrame plugins : ignore
it! (was an issue in one example plugin, not general)