This is a little bit icky, but I can't see a nicer way to do it.

Split giant methods up, simplify logic in a few places.

From everywhere.

I'm planning on removing scoped IconManagers - it's a lot of effort
to maintain them and pass them in everywhere (instead of injecting
them) for almost no gain. I don't think we've ever set a per-server
or per-channel icon, and I can't imagine why we'd want to.

This is horribly icky and wrong. The only place it's used is
PluginFilesHelper, which can easily be given its own manager.

Also fix annoying shaded import.

Alternative highlights were never added because the maps were
optimised away; then the list occasionally ended up with null
entries as they were added concurrently.

For some reason it reordered the ctor parameters, so fix them too...

Doesn't make sense to have this any more, as Chat provides the
common interface between queries + channels.