No user visible changes expected.
This is needed for the asset shelf (#104831), so that the user can
resize the asset shelf region, but it's ensured to always be snapped to
a multiple of the row height (which can change over redraws).
Before this, `RGN_FLAG_DYNAMIC_SIZE` would have to be set so that
regions can control their own size in the `ARegionType::layout()`
callback. But this would also disable resizing the region by the user.
Tagging regions as being dynamically sized and disabling user resizing
are now two separate options/flags.
Included changes:
- Rename `RGN_FLAG_PREFSIZE_OR_HIDDEN` to `RGN_FLAG_NO_USER_RESIZE` and
make that generally disable user resizing like `RGN_FLAG_DYNAMIC_SIZE`
used to, so that it can be used for more than just the properties
editor tabs region.
- Ensure regions that relied on the previous `RGN_FLAG_DYNAMIC_SIZE`
behavior that disallowed user resizing have the
`RGN_FLAG_NO_USER_RESIZE` flag set too now.
- Versioning to ensure the previous point for old files.
- Update comments.
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