GLMixer Code

Improved appearance of Gamma Levels dialog
+ fix compilation warning in video recorder

Fix bug with gamma levels widget (gamma was changing every time a source was clicked).

Added QLogStream to also log the std::cerr and std::cout messages.
+ tiny corrections.

Bug fix (and reverse back) of message Handler ; it caused program crash on system Critical messages.
Added preference to use native file dialogs for QFileDialogs.
Improved appearance of Log window.

Uniform icon size to 24x24 + update some icons.
Bug fix Clone (messed up in previous commit) & in catalog context menu.

Fixed bug in reset Source action.

Bug fix in clone source log.
Improvements in GammaWidget interaction (replaced slider by direct plot manipulation).
Various fixes in management of GLMixer static singleton instance.

Tiny bug fix and visual improvement in logging.

Implemented clean exit for new singleton instance mechanism of GLMixer.

Made GLMixer instance singleton and using Qt::QueuedConnection slot calling to forward messages handled by QtMsgHandler to the GLMixer single instance ; calling qDebug() (and others) to fill in GLMixer logs is now thread safe.