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
(cherry picked from commit 724f10a895)
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.
