Move Mk/bsd.gnome.mk and Mk/bsd.mate.mk to Mk/Uses/.

To use the GNOME or MATE components activate it by adding gnome
or mate to USES. The usage of USE_GNOME/INSTALL_ICONS and for
example GLIB_SCHEMAS has stayed the same.

Like with USES, the use of USE_GNOME and so after bsd.port.pre.mk
is now forbidden. And adapt ports that where still doing that.

Exp-runs done by: antoine@

PR: 205432
Reviewed by: antoine@, mat@
Approved by: portmgr (antoine@)
Differential Revision: https://reviews.freebsd.org/D3653