D7714: Show dialog to ask when closing when more than tab open

[Nathaniel Graham]

ngraham added reviewers: Okular, KDE Applications.

REPOSITORY
  R223 Okular

REVISION DETAIL
  https://phabricator.kde.org/D7714

To: aacid, #okular, #kde_applications
Cc: colomar, rkflx, #okular, aacid