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: I5b23b02151c75bb49831a2bbf8d515eb0b54415e
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/199068
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
Tested-by: Jenkins