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).
