qml: merge MainContext and MainInterface
The rational for this change is:
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MainContext doesn't have much value, most things it contains can be accessed through the MainInterface object.
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MainInterface is no longer the interface with the QWidget meaning, but it's used as a context.
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I needed Network models to access our MediaLib instance rather than directly the vlc_medialibrary_t instance for a future MR
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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