Not every instance of a Filterable implementation is supposed to be
offered as a search criterion through the Dropdown. So far, this
distinction was simple - all instances of a certain implemention were
either offered or not, and determined by Dropdown through the
corresponding BaseManager.

However, a subsequent commit will introduce GMount-backed
PhotosSources, which shouldn't be presented through the Dropdown,
while other Sources should be. This method will help with that.

#29