GTK GUI: options: support for disabling option buttons

[Martin Kolman]

• support for disabling option buttons is needed

• usecase:
  • Monav offline routing currently does not support avoiding toll and highways
  • therefore, the corresponding buttons should be disabled when Monav routing is active

• as not to confuse the users, disabled buttons should not be visible
  • as modRana has its own graphical style, it wont be easily possible to just gray them out as in normal GUI toolkits, as this might confuse the users