Move the enum class out of weld.hxx into its own header, in
order to make it easier to keep an overview and to prepare
for getting rid of weld.hxx altogether.
Change-Id: Ie6cd28d7cc6965c955f526da0d89adbfba62f525
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/205926
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
Tested-by: Jenkins