qt/qml: Add 'Group by folder' in the Video(s) main view

This MR adds "Group by folder" support in the "Video / All" main view. Now the user can decide wether he wants a "flattened", "grouped" or "grouped by folder" representation of the corresponding MediaLibrary model(s).

This MR is a sequel to the previous "qt/qml: Merge groups inside the Video(s) main view" contribution: !1041 (merged)

The MediaLibrary should be updated soon to support the folder duration field and update events:

• medialibrary#409 (closed)
• medialibrary#410 (closed)