Use file mode in Preview add-on

Summary:
- prefer file mode over content-based mime type
- update preview on update of file mode and change of url
- fix crashes when switching views with opened preview

Test Plan:
- start empty kate session
- enable preview add-on
- open previewable document to replace empty default document
  * with patch the preview is displayed
- change the file mode to Normal
  * preview is cleared
- restore the original file mode
  * preview is displayed again
- close file
  * preview is cleared
- open a few previewable files and lock one of them, switch tabs
- close the locked file
  * previous behaviour with locked previews is maintained

Reviewers: #kate, kossebau, cullmann

Reviewed By: #kate, cullmann

Subscribers: cullmann, dhaumann, kwrite-devel

Tags: #kate

Differential Revision: https://phabricator.kde.org/D12131