GLMixer Code

Fixed loading order of FreeFrame and Shadertoy plugins (could benefit
from upgrade version of GLM file). Linking Undo Manager to GLMixer
qApplication to follow-up on status of the session (and ask to save if
there was a change).

Update Property Browser and Selection after undo or redo.

First (sort of) working implementation of Undo Manager.

Fixed Preference and Settings issues (OSX)

Enable forced reload on web page stop.

Fixed update of Web Source.

Minor fix (compilation warnings, warning messages at runtime, etc).

Implementation of scaling and rotation of sources in Mixing view ready
for release. Added parameter for size of icons in mixing view and
improved icons for tools.

Updated readme