roomList: Show indicator while channel is ensured

After connecting to the network, rooms often take some time before
they are joined. Show a "content-loading" symbol next to each room
if the room takes more than 3 seconds to ensure.

https://bugzilla.gnome.org/show_bug.cgi?id=775239
1 job for wip/fmuellner/room-progress in 2 minutes and 48 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Build
passed build-polari #1788

00:02:48