To avoid continuous height changes while browsing through categories,
we let the list of categories determine the overall height, and rely
on scrolling for the list of apps within a category.
We currently achieve this by assigning a fixed height via the
`style` property. This has been found to trigger a crash when
running headless, as we end up querying an actor's height request
before a valid resource scale is available.
Instead, use a custom layout manager, which seems more elegant anyway.
Close: https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/issues/472
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/290>
(cherry picked from commit 5652182fb3)
6bbc329e7b