Implement GroupDialog and CurriculumDialog.

Five down, one (and a half) to go.
Also, i've got very fat controllers on these guys.
Refactoring improvements on 'editor'.

Add docstrings in base dialog class.
Redefined default size of dialog.
Renamed some vars in SubjectDialog.
Renamed DBEditorWindow method update_table() to update_table_view().
Implemented Instructor-Subject table and corresponding dialog.