qml: Add a "show containing directory" entry in contextual menu of medialibrary items

On medialibrary items we should add a contextual menu that allows to open the folder containing the media in the system explorer, We already do this for the playlist, see playlist_controller.cpp

some notes:

Code of the contextual menu for ML items is located in MLContextMenu.qml
It is possible to open URL with external application using QDesktopService from C++ or qt.openUrlExternally from QML
We should open the containing folder, not the URL itself
We should probably only propose this for local files
We should propose this options for media that have a url/mrl (a genre doesn't have an url)
We shouldn't open folder for more than one item (don't launch 50 browsers)

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