GLMixer Code

Fix bug session manager (reload list after creation of a new session)

Code cleanup (deprecated Q_SIGNALS and Q_SLOTS)

Simplified opening of session (no busy indication, all events processed
before, etc.)

Closing properly Video File, and adding mechanisms to force thread
termination.

Added the type of the source as first specific property.

Compiling the undo history in rendering manager only if
HISTORY_MANAGEMENT flag is enabled.

Disabling use of avcodec_close as it locks the program (not thread
safe?). TODO: find a more elegant way to clean the codec.

Modularization of compilation with Session manager, Tags manager,
history manager and logger. Useful for debugging and compiling release
without unfinished features.