x11/cde-devel: Introduce x11/cde-devel

The x11/cde-devel port will track the CDE master development branch.
The cdesktopenv-code git repo on sourceforge is mirrored using my github
account and FreeBSD ports plumbing is used to fetch tarballs from github.

This port uses the x11/cde-25 port/pkg coexistence methodology to allow
it to be installed while a more stable version of x11/cde* is installed,
allowing users to safely use the port with a quick fallback should the
upstream software significanly break.

Users should ensure that their ~/.dt is backed up prior to switching
from a stable version of x11/cde* to x11/cde-devel or back again.