Copy raster image and SVG content to clipboard

Summary:
The normal Copy/Cut options (Ctrl+C/Ctrl+X) copy the
url(s) of the selected file(s)/folder(s) to the clipboard.
This patch additionally copies raster image and SVG content of the
current document to the clipboard. In Browse Mode the content is
copied only if a single file is selected.

FEATURE: 290294

Test Plan:

1. Open raster image or SVG in Gwenview
2. Edit → Copy or Ctrl+C to copy image to clipboard
3. Paste image to different applications (e.g. gimp, office writer, facebook chat in web browser, inkscape, etc.)
4. Cut&Paste should only move the file if pasted to file manager applications (e.g. dolphin)

Reviewers: Gwenview, rkflx

Reviewed By: Gwenview, rkflx

Subscribers: huoni, rkflx, ngraham

Differential Revision: https://phabricator.kde.org/D12871