VOL-54: Refactored getProfileIDs() method, using uf_join directly instead of by way of multiform, and caching results in a property.
VOL-54: Moved responsibility of building profile widgets from setDefaults() method to preProcess() method, relying on getProfileIDs() instead of multiform. Behavior change of note: uf_joins are no longer created automatically on loading the form if the project has no profiles set (instead a widget form element is prepopulated with CiviVolunteer's reserved profile).
VOL-54: Encapsulated post-processing code related to profile selection in saveProfileSelections() method, working with uf_join directly rather than by way of multiform.