qml: merge MainContext and MainInterface

The rational for this change is:

• MainContext doesn't have much value, most things it contains can be accessed through the MainInterface object.

• MainInterface is no longer the interface with the QWidget meaning, but it's used as a context.

• I needed Network models to access our MediaLib instance rather than directly the vlc_medialibrary_t instance for a future MR

• using a QML singleton rather than a context property should provide a more efficient name resolution.

Note that I can live without the renaming if you don't like it.

Tests are welcomes, I didn't see any regression, but it touches pretty much every QML files