Fixed custom mode + 2 small additions

The custom mode wasn't working properly because there was still a check for 
*self.medianame* instead of *self.dbid*.

I also added an option to show the image selection directly when using gui mode. E.g.

*RunScript(script.artwork.downloader,mode=gui,mediatype=movie,dbid=$INFO[ListItem.DBID],poster)*

will directly open the image selection for type 'poster'.

I also added a setting which disables the ok dialog and "downloaded: 0..." notification if the user doesn't choose an image and closes the dialog.