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)

common/flatpak-dir.c

@@ -3113,6 +3113,9 @@ export_desktop_file (const char   *app,
                                       "X-Flatpak-Tags",
                                       (const char * const *) tags, length);
         }
+
+      /* Add a marker so consumers can easily find out that this launches a sandbox */
+      g_key_file_set_string (keyfile, "Desktop Entry", "X-Flatpak", app);
     }
 
   groups = g_key_file_get_groups (keyfile, NULL);