Minor fixes for 'scheduler', DB populator and 'editor' dialogs.

Make DB populating process more realistic.
Refactor GenericUpdateDialog a bit.
Rename a bunch of methods in SchedulerWindow.
Add ordering for subjects and instructors.
Implement a few useful methods in SchedulePerDayView.
Implement dialog for editing pair in schedule for 'scheduler'.
Another refactoring.

Rename schedule_model_update method in SchedulerWindow.
Remove debug output.
Implement deletion on schedules.

Define appropriate methods in schedule model.
Define generic method schedule_model_delete().
Define handlers for buttons for needed actions:
 - delete selected row from schedule
 - delete schedule for selected group
 - delete whole schedule