SvLBoxButton and SvLBoxButtonData are only used inside vcl.
Move them from a global vcl header to a vcl-internal header
and to an own source file named after the new header file.
Change-Id: I118eeb724264fdde7421df163af1a37cb14d5d56
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/206317
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>