D9328: Add support for cancellable image rendering and text extraction

[Albert Astals Cid]

aacid planned changes to this revision.

REPOSITORY
  R223 Okular

REVISION DETAIL
  https://phabricator.kde.org/D9328

To: aacid, ervin
Cc: rkflx, ervin, michaelweghorn, ngraham, #okular, aacid