If "Region Overlap" was disabled, the region coordinate calculations
would end up placing the header on the bottom, taking space away from
the main asset shelf region. This is because the header was abusing the
`RGN_SPLIT_PREV` flag a bit, because it gave the desired behavior of
hiding the regions together. This worked well because the regions
wouldn't actually do the splitting when one used region overlap and the
other not. But disabling region overlap would bring the splitting
usually enabled with the `RGN_SPLIT_PREV` flag.
This PR removes the `RGN_SPLIT_PREV` flag from the asset shelf header.
Instead a new flag is introduced to only enable the behavior of
`RGN_SPLIT_PREV` of hiding a region together with the previous one. So
the header stays at the top of the region, and labels stay visible.
The new flag is added to the `ARegion.alignment` flags, even though it's
more related to behavior than alignment. However this is more
convenient, and keeps it together with related flags.
4f0258f549