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 
"These classes " (also line 37: "This classes must implement" > "These classes...") "can define an optional 
``supported_apps`` attribute, specifying which applications the toolbar will work with. This is useful when the 
toolbar is defined in a different application from the views it's related to.