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