gnome-extra/gnome-dvb-daemon: EAPI6, python-single, py37, other fixes

gnome-extra/gnome-dvb-daemon: EAPI6, python-single, py37, other fixes

* Port to EAPI-6 and drop some unnecessary eclass inherits
* The client code is all in python with shipped modules, so it has
  to be python-single-r1 instead of python-any-r1 (this happened
  to be fine with any due to only one python version supported
* Add python-3.7 support
* Drop totem plugin, as it can't function with in-tree totem anymore,
  because it doesn't support plugins changing the UI (in addition,
  the totem plugin was py2-only). Upstream has a grilo plugin now
  instead, but it has never been released before the whole project
  has fallen into de facto unmaintained status
* Drop useless USE=nls
* Drop USE=vala that doesn't do anything (the pregenerated C files
  are always used)
* Fix python module optimization handling
* Drop questionable libltdl dep
* Drop perl dep - it appears only used via intltool
* Standard subslot depend for gobject-introspection

In general I can't get this project to do anything useful, so help
maintaining and/or testing this welcome, if it has any actual users.
Or even letting me know how to get this going properly (I do have
a DVB-T USB device and some terrestial channels available).

Closes: https://bugs.gentoo.org/701202
Package-Manager: Portage-2.3.84, Repoman-2.3.20
Signed-off-by: Mart Raudsepp <leio@gentoo.org>