Reinstate support for GTK2 (rather than GTK3) as an OPTION.
While here, add a putty-gtk2 slave port and update _CONFLICTS.
Since the default package does not change, and pkg would handle conflicts
around attempted installation of the new -gtk2 package, omit the
PORTREVISION bump from Miroslav's original patch.
PR: 227200
Submitted by: Miroslav Lachman