Add method to PermissionSystem for retrieving user groups

[Ryan J Ollos]

I have wanted a method of PermissionSystem for retrieving all groups that a user belongs to, both those defined in the permissions table and in IPermissionGroupProviders.

[Ryan J Ollos]

Proposed changes in [3fdd6a387/rjollos.git]. Example use: privateticketsplugin.diff​.

1. get_user_permissions​ is similarly named, but returns a dictionary. I have never understood the usefulness of mapping all the keys to True, so I chose to return a sorted list from get_user_groups.
2. get_user_permissions returns all permissions when username is None. The same can't easily be done for get_user_groups because IPermissionGroupProviders don't return all groups when username is None. I'm not even sure we'd want that behavior for DefaultPermissionGroupProvider​. We'd probably need to add a get_all_groups method to the IPermissionGroupProvider interface.
3. get_groups_dict​ doesn't really return all groups, because it doesn't account for IPermissionGroupProviders. We'd probably need the DefaultPermissionGroupProvider.get_all_groups method for it to return all groups. This could be considered a defect in group expansion​ of the set_owner field.

[Ryan J Ollos]

Committed to trunk in r16326.