[FR]: Allow the addition of "Horizontal Rule" UI element in sources sidebar.

[thupsi]

Sometimes when I organize sources in the sidebar I would like a lighter separator than an actual heading, yet more defined than a blank heading (that just leaves empty space). An option (like in the FLD) to add a "Horizontal Rule" would be just the ticket.

IMO it would be especialy handy when we want to separate solitary sources, in which case a group heading looks awkward.

It would then look something like this (Sorry, it's in Greek, but you get the idea):