[Merge] lp:~fboucault/camera-app/hook_up_hdr into lp:camera-app

[Florian Boucault]

This can be merged independently but the feature will not work until the following MR has landed: https://code.launchpad.net/~fboucault/qtubuntu-camera/hdr_scene_mode/+merge/225993
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https://code.launchpad.net/~fboucault/camera-app/hook_up_hdr/+merge/226219
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