qt/qml: Merge groups inside the Video(s) main view

This MR merges the group views inside the "Video / All" main view. Now the user can decide wether he wants a "flattened" or "grouped" representation of the corresponding MediaLibrary model(s).

Currently, the group option supports "Group by name" and we need to improve our MediaLibrary wrapper to handle the "Group by folder" option. This will come in a subsequent patch. After this our views will match what we traditionnaly finds on the Android application, with the exception of defaulting to the flattened view, because the world might not be ready.

This MR also updates the group models to make sure they are on par with the video models.