Improved confirmation dialog shown to the user before discarding changes

Improved confirmation dialog shown to the user before discarding changes

- Split the confirmation dialog's text into a primary and secondary
  section.
- Changed the button label to "Cancel" and "Discard" instead of "Yes"
  and "No".
- Added a accounts_dialog_show_question_dialog() function to show
  question dialogs related to the accounts dialog and connect to its
  "response" signal.
- Removed the empathy_show_yes_no_question_dialog() function since it
  was added for this specific patch but is no longer used.