just a quick suggestion: wouldn't it be nice to have the number of songs displayed instead of a simple boolean-ish icon when using Queue in a layout?
maybe it could be something like Queue(with_text=1)?

Yes, I wanted to add this when I added the with_text option, but I'm not sure what text should be displayed.
The easiest is probably to use the first line of the tooltip.
I plan to add a way to customize the text.

By the way, I'm thinking of swapping the left-click / right-click actions of the Queue/Filter/Sort widgets, the left-click is a currently a bit destructive, and the right-click is probably not tried by many.
Any thoughts ?

my suggestion would be:
"Queue: $n"

i would make the left click show a menu. (i know that there's the queue-menu already) i guess adding a "clear queue" and "show queue" button to that menu would make that more accessible (not all layouts have a queue-pane).
maybe the right-click (or middle-click?) could be something like clearing the queue.

the same goes for the sort and filter widgets, i would suggest making them menus but with a dynamic title. that would also reduce the visual clutter a bit imho.