x11/budgie: + Budgie desktop environment.

This is import of complete Budgie desktop, it contains:

  - Uses 'budgie.mk' framework
  - Add virtual category (budgie) in `Mk/bsd.port.mk`
  - Meta-port `x11-wm/budgie`
  - New icon theme `x11-themes/tela-icon-theme`

Reviewed by: rene, tcberner, lbartoletti, arrowd

Differential Revision: https://reviews.freebsd.org/D37224