[Merge] lp:~thomas-voss/qtubuntu-media/fix-races-for-access-to-destroyed-controls into lp:qtubuntu-media

[Alberto Aguirre]

Does not seem to fix #1378311 with the example.qml attached in that bug description.
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https://code.launchpad.net/~thomas-voss/qtubuntu-media/fix-races-for-access-to-destroyed-controls/+merge/238404
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