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
Introduce a HmenuItemPopupSpawner component that takes care of reacting
to onTriggered signals and calling the utils.makePopup() function.
The combination of this new component used in HMenu to open a HPopup
now correctly takes and restore focus to the right element when closed.
