Allow Gwenview to be able to open .kra (Krita) files

Summary:
Add support for opening Krita files.

FEATURE: 337272
FIXED-IN: 19.04.0

Depends on D19120

Test Plan:
- Apply the patch and then this one for Krita: D19120
- Rebuild and install both apps
- Run `sudo update-desktop-database`
- Right-click on a .kra file in Dolphin. Krita shows up as the top app to open it, with Gwenview showing up in the "Open With" menu

Reviewers: #gwenview, sitter, cfeck

Reviewed By: sitter

Tags: #gwenview

Differential Revision: https://phabricator.kde.org/D19121