Mark a song edited from preview as coming from plugin. Fixes bug 1382672

Fixes: https://launchpad.net/bugs/1382672

openlp/plugins/songs/lib/mediaitem.py

@@ -339,6 +339,9 @@ class SongMediaItem(MediaManagerItem):
                 self.remote_song = -1
                 self.remote_triggered = None
                 if item:
+                    if preview:
+                        # A song can only be edited if it comes from plugin, so we set it again for the new item.
+                        item.from_plugin = True
                     return item
         return None