Avoid data loss when importing pictures

Summary:
Fix porting regressions, which left users of Gwenview Importer with:
- failed import (import destination still empty)
- additionally (when choosing "Delete" instead of "Keep" after import):
  pictures also removed from import source, with no way to recover

Correct additional problems remaining after fixing the import failure:
- hang on duplicate filenames
- identically named files with different content are never imported
- error dialog when deleting pictures from import source

BUG: 379615

In detail:

1st problem (introduced in 017b4fe5):

  Initially, pictures are copied to a temporary folder
  (e.g. "foo/.gwenview_importer-IcQqvo/"), before being moved/renamed
  to the final destination (e.g. "foo/"), which is determined by
  calling "cd .." on the temporary folder.

  However, mistakenly this path contains a superfluous '/'
  (e.g. "foo/.gwenview_importer-IcQqvo//"), resulting in the final
  destination reading "foo/.gwenview_importer-IcQqvo/" instead of
  "foo/". On cleanup, the temporary folder is removed, simultaneously
  deleting all new pictures.

  Fix: Omit '/' where appropriate.

2nd problem (broken by a3262e9b):

  When trying to find a unique filename, the while loop "stat"ing the
  file repeats forever.

  Fix: Call "KIO::stat" and "KJobWidgets::setWindow"
       also inside the loop.

3rd problem (introduced in 017b4fe5):

  If the destination directory already contains an identically named
  file, we try to find a different name by appending a number. For
  this, we need to strip the old filename from the full path.
  Unfortunately, only calling "path()" is incorrect, giving
  "foo/pict0001.jpg/pict0001_1.jpg", rather than "foo/pict0001_1.jpg".

  Fix: Use "adjusted(QUrl::RemoveFilename)".

4th problem (broken by a3262e9b):

  Although deletion works, we show a warning dialog stating failure.

  Fix: Invert the logic of "job->exec()", as the error handling should
       be skipped by "break"ing out of the while loop.

Test Plan:
Steps to reproduce (see https://bugs.kde.org/show_bug.cgi?id=379615) no longer result in data loss.

Autotests for importer (separate review request in D5750) pass. Hopefully, this helps catching any future porting regressions.

Reviewers: ltoscano, sandsmark, gateau

Reviewed By: ltoscano

Differential Revision: https://phabricator.kde.org/D5749