Browse dialog: indicate and remember hidden list servers

[askmeaboutlo0m]

In the Browse page of the start dialog, you can double-click on a list server to hide them. It would be nice if Drawpile would remember this persistently, but this requires adding some kind of indicator that a list server is hidden, otherwise it just looks like there's no sessions underneath it. Ideally it should have the same kind of triangle on the side that layer groups do.