GLMixer Code

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).

Reference all plugin (FreeFrame & Shadertoy) as GPU Plugin in GUI, labels and icons.

New implementation of Copy-Paste mechanism; this allows copying
source(s) and pasting them back in another session, or into the same
(then sources are cloned). Pasting of images from clipboard replaces the
creation of capture source (which is still possible by capturing the
output and pasting it). GUI for source creation of capture source
changed accordingly.

Slight change of behavior in mixing : plugins are not stopped when a
source is stopped (for uniformity of behavior with non-playable source),
and sources are set to stanby mode even when cloned (this was not clear
why a source would not behave the same).

BUg Fix; correctly connecting play button to source status (playing, not
playing, or standingby).