Implement CMSToolbar.supported_apps

This fix is to overcome the troubles we have to get the "right" application name when populating ``is_current_app``
By adding a "manual override" we can bypass application mismatch or impossibile matches.
Action is required only for those cases for which the current strategy doesn't work, so it's totally backward compatible.
Targeting support because it's a bugfix, not a new feature
Fixes #3520 
I just corrected the typo s/propagate/populate/ but I agree that it needs further clarification