Make some file formats return KisImportExportFilter::UserCancelled when their options dialogs are cancelled

Summary:
The file formats affected by this patch are: r16 heightmap/r8 heightmap/JPEG-2000/JPEG/PNG/PPM/PGM/PBM

These formats return KisImportExportFilter::OK rather than KisImportExportFilter::UserCancelled when their import/export options dialog is cancelled.

This leads to buggy behaviour that can be reproduced using either set of the following steps:
CASE 1
    * Create a new document and modify it
    * Close the document and opt to save it as one of the affected formats
    * Cancel when the export options appear
    * (The document closes without saving)
CASE 2
    * Open a heightmap
    * Cancel when the import options appear
    * (Krita crashes)

I've also modified kis_png_export.cc so that it sets up the Cancel/OK buttons explicitly instead of relying on getting them by default from KDialog.

Reviewers: #krita:_stable, rempt

Reviewed By: #krita:_stable, rempt

Subscribers: rempt

Differential Revision: https://phabricator.kde.org/D548

Patch by Nicholas LaPointe, thanks!