Add indication of nested priority items

Right now there is no indication if a regular priority item contains a high priority item underneath it, unless the entire tree is expanded.

It would be great to get an indicator showing that an item has some priority items underneath it so they don't get lost or ignored.

This could perhaps work like the nested update dot works. There is a solid dot if an item has an update, and a lighter outline dot if a parent contains an item with an update.