sd theme: add UI (area dialog) for shape fill color
SvxColorTabPage::FillItemSet() has to produce a color item that has the
theme index, which means SvxColorTabPage::SelectValSetHdl_Impl() has to
change the current color to an svx::NamedThemedColor.
The rest is just fallout from this, now that the current color has
theming metadata.
Change-Id: If0018c651239ba74f45745e69e41ff7040ac9b97
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/132327
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>