D20678: Ensure compatibility with Qt <5.10

[Albert Astals Cid]

aacid accepted this revision.
This revision is now accepted and ready to land.

REPOSITORY
  R223 Okular

REVISION DETAIL
  https://phabricator.kde.org/D20678

To: awilcox, aacid
Cc: okular-devel, joaonetto, tfella, ngraham, darcyshen, aacid