Added mutex to Action manager for access to undo history and added mutex to session's snapshots. Lock and unlock those mutex for all write access and long read access.
As in update of Mixer manager, failed sources are managed in update of SessionSources. RenderSources that fail are re-created. Attach/Detach/Attached of sources functions is unified between Session and Mixer.
Session stores list of all callback instances and reacts on input release by calling the reverse callback if it exists, or by finishing the ongoing callback. This means the behavior of Callbacks is different for those who are reversible (i.e. returns a non-null reverse) from those which do not have reverse. The reversible callbacks enforce to be exclusive while active (key pressed), others can be repeated and complementary (run in parallel).
Session contains a set of 'Batch' that are created in the Player (renamed from PlayGroups). Session InputCallback can now target either a Source or a Batch, using std::variant (new type Target). Input Mapping reacts to input to create callbacks to a target, either a single source (as before) or to a Batch (multiple sources).
When a source in a session fails, it is not anymore deleted after update; the Mixer keeps it in the session but detaches it from the scene. This way the user can access the failed source in the navigator (listed in RED), and Replace the source. The Replacement of source is available for any source. The source visitor does not skip a visit if the source failed.
