sidebar: improve invalidation tracking for Panels.
Only emit an invalidation on panels that change position.
Ensure we invalidate the ScrollContainerWindow for updated separators.
Emit a single vcl::Region for an invalidation rather than per panel.
Change-Id: I5452791ac9a7d1a9b8604c7704d24641160c275b
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/86234
Tested-by: Jenkins CollaboraOffice <jenkinscollaboraoffice@gmail.com>
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/86288
Tested-by: Jenkins