Move the class out of weld.hxx into its own header, in
order to make it easier to keep an overview and to avoid
having to recompile ~all UI code when the class is
modified.
Change-Id: Ide86f35543468a88288a068cc9f89eaf381ac52f
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/205763
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>