x11-wm/e16: Add E16 back to the tree

x11-wm/e16: Add E16 back to the tree

Enligtenment DR16 (e16) was earlier packaged as
x11-wm/enlightenment:0. No package move is possible here, since
the slot is being extracted and added as the separate package.

It is based on e16-overlay by Scott McClung <scott mcclung.com>:
https://github.com/mcclung/e16-overlay
However, there are many changes:

A plethora of new USE flags is added: e16 is highly configurable
with many yummy features. Defaults are set to some reasonable, but
smaller number of enabled features than upstream does.

Users can now provide custom fonts using ~/.e16/fonts/ and
~/.e16/fonts.cfg.

EAPI is updated to 7 with whole ebuild overhaul following.

Bug: https://bugs.gentoo.org/658860
Package-Manager: Portage-2.3.62, Repoman-2.3.12
Signed-off-by: Andrew Savchenko <bircoph@gentoo.org>