Give warnings when the file is modified externally

Summary:
Unfortunately, poppler (the only backed that supports saving) is not able
to save properly if the file is modified by a third party while it is opened

So we give the user a warning saying things went wrong and give him the option
to not reload/close, that way if there was something very important in the annotations
she added she can try to save them (even if by copy&paste the contents to a third program)

Reviewers: rkflx

Reviewed By: rkflx

Subscribers: ngraham, rkflx, ltoscano, #okular

Tags: #okular

Differential Revision: https://phabricator.kde.org/D8863