Add a marker to rewritten desktop files

Add X-Flatpak=$app to rewritten desktop files.
Desktop files have multiple consumers, and this
makes it easier for them to know what to do.

(cherry picked from commit 66e91f55e8
 and e75cff6bb5)
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f