mail/claws: Rework claws-mail-plugins

Let mail/claws-mail-plugins get PORTVERSION from mail/claws-mail.

There is PORTVERSION defined directly in
mail/claws-mail-plugins/Makefile at present. Create
mail/claws-mail/Makefile.ver to hold this information and use it in both
mail/claws-mail-plugins/Makefile and mail/claws-mail/Makefile.claws
files.

Technically, it is a no change for now, but it makes easier to create
new options for both GTK2 and GTK3 (new one) version of Claws Mail and
act accordingly in all Claws Mail plugin ports.

Approved by: Chris Hutchinson (maintainer)
PR: 266092