GLMixer Code

Redesign of MixingToolboxWidget Presets. Now using presets described as
XML, and applying configuration to source using xml. This enables saving
GPU Plugins in presets. This unifies the use of presets as 'property
brush'.

BugFix : inversion of ShaderToy plugin when applying FreeFrame plugin
configuration on a clone source.

Cosmetic cleanup icons & a bit of fun.

Sorting out icons for preference, presets, gear, etc..

Updated icons for presets.

Bugs Fix copy-pasting clone sources with shadertoy plugins. More robust
Clone source setting of original.

Updated icons in user preference (following changes from copy-paste icons).

Implemented CUT action. Added icons for actions copy, paste, cut, undo &
redo. Fixed Workspace for paste action.

Enabling undo of paste action.

Unified Media file extension for opening file dialog (selection of video vs. image formats was not obviously accessible in OSX, and it is anyway useless to distinguish as the user can open both).