Indicate that Gwenview can open Targa images

Summary: BUG: 332485

Test Plan: After updating the .desktop files and running `sudo update-desktop-database`, Gwenview was correctly registered as a handler for .tga files, and they opened and displayed exactly as they should.

Reviewers: #kde_applications, rkflx, broulik

Reviewed By: rkflx

Differential Revision: https://phabricator.kde.org/D8751