Show dialog to ask when closing when more than tab open

Summary:
The checkbox is checked and says "Warn me on closing more than one tab",
for that reason we can't use the default KMessageBox::questionYesNo since
there the checkbox is always not checked and it's when checked that you enable it

Inspired by code from torham zed torhamzed@yahoo.com at review request 126406

Reviewers: #okular, #kde_applications

Subscribers: alexeymin, ngraham, colomar, rkflx, #okular

Tags: #okular

Differential Revision: https://phabricator.kde.org/D7714