Refactor dialog system in 'editor'.

Defined base class GenericInsertDialog for inheriting it in other dialogs.
Added validation template and other things.
Also, renamed 'editor's 'main' module to 'gui'. Now i can start 'editor'
with cute command 'python -m college.editor.gui'.
Add possibility to delete table entries.

Deleting entry from curricula is a horrifying process.
(Workaround with curriculum.id is used.)
Add possibility to edit entries in tables.

Also, bind this action to a double-click on entry.
Also, bind menu button 'Quit' to real quit.
Add a bunch of new dialogs.

Renamed GenericInsertDialog to GenericUpdateDialog.
Implemented SubjectDialog and InstructorDialog.
(Three down, three to go.)