GLMixer Code

Added OSC properties to control playing of media source and videofile.

Updated the names of presets in MixingToolbox to avoid spaces so they
are compatible with OSC messages.

Added the OSC 'preset' attribute to sources : applies a preset from the
Mixing Toolbox from OSC.

Compilation fix for old G++ (before C++11).

Added OSC target 'select' to select the current source from OSC using
'next' and 'previous' attributes. Controlling the current source is then
operated using the 'current' target.

New OSC: Broadcast of message in reply to OSC request. Implemented
request for name of source and count of sources. Improved implementation
of OSC message handler.

Cosmetics GUI and code.

BugFix: react to change of number of monitors on the fullscreen of
output render window and in the user preference selection of monitor for
fullscreen.

Slight improvement of Axis cursor (added option to outline the
possibility to scroll along axis).

Created file menu action Reload Session (Shift+Ctrl+O) to quickly revert
to the latest saved version of the file. Changed terminology and shortcut
for append session: Import Session (Ctrl + I).