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
The width of the images weren't taken into consideration to calculate
the message xOffset, resulting in these messages being pushed way past
what they should be and looking very thin
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.
