"Open in new Window" goes on top, as it does in most other context menus
with similar items
"Leave room" goes on the bottom with a separator above it, because it's
a mildly destructive action.
This ports the room list delegates to Kirigami.BasicListItem leading/trailing for a more consistent appearance with other applications, and adjusts how their context menus look and behave
Previously, when there was no avatar set, the source property of Avatar was still set to 'image://mxc/',
which caused Avatar to load that from the imageprovider. The imageprovider can't provide an empty image and aborts with error
It turnout that using an QML Singleton was a bad idea, instead create a
qml object in the main component and refer to it in the rest of the
codebase.
This commit also simplify a bit the qml structure of the Menu and fix
some visual bugs.
