workspacesView: Properly allocate workspaces

workspacesView: Properly allocate workspaces

So far we've been allocating workspaces in a stack, and relied on
translation to move them to the right position. And as the position
depends on both the workspace's index and the view's viewport, some
care is needed to prevent gestures/scrolling from interfering with
layout updates.

Clean that up by properly allocating workspaces in a row or column,
and use a translation to reflect the current scroll position.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1345