GLMixer Code

Fix compilation with different cmake build directory.

Implementation of signal mechanism to detect changes in the source properties.
Using this to propose to save a session when closing if it has changed.

Avoid copy (rename) of the temporary recorded file when saving in automatic saving to folder mode.
+ remove property "attached" for shared memory source.

Edited wiki page Screenshots through web user interface.

Fixed exit crash (last hopefully).

Fixed exit crash (second).

compilation warnings fix.

Fixed exit crash.

Separation of blending equation and functions for RGB and Alpha (using glBlendEquationSeparate glBlendFunctionSeparate) to fix the bug of incorrect alpha when using inverse or custom modes (visible in feedback sources and shared memory).

Moved the display of error message box to avoid bloking the whole application for warnings.