GUI config: add support for Private ticket option

Unfortunately this option desires 3 state logic:
  - Yes
  - No
  - Ask (default state)

But system-config-abrt doesn't support 3 state logic yet.

Closes #706

Signed-off-by: Jakub Filak <jfilak@redhat.com>
Merge pull request #707 from abrt/gh706

GUI config: add support for Private ticket option