desktop file: add a Comment

Fixes #2130

mirall.desktop.in

@@ -3,6 +3,7 @@ Categories=Utility;X-SuSE-SyncUtility;
 Type=Application
 Exec=@APPLICATION_EXECUTABLE@
 Name=@APPLICATION_NAME@ desktop sync client 
+Comment=@APPLICATION_NAME@ desktop synchronization client
 GenericName=Folder Sync
 Icon=@APPLICATION_EXECUTABLE@
 Keywords=@APPLICATION_NAME@;syncing;file;sharing;