appimage: use original app desktop file as base for appimage one

Summary:
Makes sure the strings & metadata are consistent, also brings in
translations.

Reviewers: KDevelop, kfunk, brauch

Reviewed By: KDevelop, kfunk

Subscribers: kdevelop-devel

Tags: KDevelop

Differential Revision: https://phabricator.kde.org/D17027