In the plasmoid:
Convert plasmoid to a handler from an observer
Support delegate channels (via a nasty hack currently)
Add popout button to delegate to main text UI
Merge the conversationsModel and handler into one class
Fix exposing a load of items to QML for no reason.
Some additional tidying. (and some additional messying :-) )
Clear unread messages on TextChannel::pendingMessagesRemoved()

In the textUI:
Always raise the window in the main Text UI handleChannel unless
 hint explicitly says otherwise.
Additionally fixes a bug in which reconnecting an account could
cause the tabs to 'randomly' switch.

REVIEW: 103908
CCBUG: 293526