D8863: Give warnings when the file is modified externally

[Albert Astals Cid]

aacid added subscribers: ltoscano, rkflx.

REPOSITORY
  R223 Okular

REVISION DETAIL
  https://phabricator.kde.org/D8863

To: aacid
Cc: rkflx, ltoscano, #okular, ngraham, aacid