Add option to hide professor's picture

Preferences.cpp
galaxql.cpp
galaxql.h

- New preference 'ShowProfessor'
0: hide
1: show (default)

(RestorePreferences)
- Restore 'ShowProfessor' preference.

(Galaxql::CreateControls)
- Change menu 'Graphics' to a more general menu 'Appearance'.
- Add an entry in 'Appearance' to toggle the visibility of the
professor.

(Galaxql::OnShowProfessorClick) New.
- Toggle the visibility of the professor (show or hide).
- Save the new visibility state as preference 'ShowProfessor'.