macosx: Add horizontal expand type for supplementary detail views in horizontal library collection views

At the moment in master, the supplementary detail views are inaccessible in the horizontal video carousels in the video library as the view is too vertically constrained to make space for them.

Expanding the carousel vertically is both inelegant and bug-ridden due to NSCollectionViewLayout's internal sizing and placing procedures.

A better solution is to add support for horizontally-opening supplementary views when the collection view layout is set horizontally. This is what this PR introduces:


Signed-off-by: Claudio Cambra

Edited by Claudio Cambra

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