Enable use of a source as mask for another source. Improved Mask mechanism in Source class, with various flags for update of source (avoid repeated mask update (GPU costly). Using SourceLink to link source to mask (improved robustness of SourceLink).
Rename class Glyph in Decoration to Character ('Glyph' was used in system X typedef). Added MouseOver in Mixing and Layers view to show highlight color on icons of Symbol and Initials of source. Show left panel on clic on source Initials.
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).
