Use native ListModel which require a lot of changes, but should be
much faster than the old way which exponentially slowed down to a crawl.
Also fix some popup bugs (leave/forget).
Not working yet: side pane keyboard controls, proper highlight,
room & member filtering, local echo replacement
Replace broken old utils.debug(...) (since the utils.js → Utils.qml
transition) with a declarative approach:
a console can now be opened with by placing a `DebugConsoleLoader {}`
anywhere.
mainUI has a default console opened by F1/alt-shit-c when no other
console is loaded.
