Refine behaviour for dragging modified images to external applications

Summary:
In f365a38fa28f support for copying modified images to external
applications was added. With the refactoring in D13248, we can extend
that support for dragging.

Depends on D13248

Test Plan:
Start dragging in Gwenview and drop onto the target for every
combination of:

• Source: Browse mode, Thumbnail Bar (normal and fullscreen), Start → Recent Files
• Formats: JPG, PNG, NEF, SVG
• Regular and Cropped image for every source file format
• Target: Dolphin, GIMP, LibreOffice, Inkscape

Only dragging of modified images to Dolphin is known to be broken
currently, but no regression either.

Reviewers: Gwenview, muhlenpfordt

Reviewed By: Gwenview, muhlenpfordt

Differential Revision: https://phabricator.kde.org/D13249