Folders reflow across pages because they don't set
allow_incomplete_pages to true. This means we want the nudging of items
to happen slightly differently when dragging an item across pages:
- When dragging from lower page index to a higher one, always reflow
towards the start of the grid (because there's now an empty slot on the
old page and items on the new page will force-reflow towards that)
- When dragging from a higher page index to a lower one, we can reflow to
the end as we usually do
To archive this, factor out the selection of "reflow direction" into a
separate variable that always defaults to "end" (because empty space is
always at the end of the grid). Set it to "start" when the item created an
empty slot on the current page or (and this is new:) on a previous page in
the folder case.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2348>
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