Add eid-mw: authenticate & sign with Belgian eID identity cards

Adds limited but useful support for Belgian electronic ID cards.

Limited for now, because:
- While a Firefox extension is included and installed, no effort is made to integrate it. I use Chromium.
- The library should work with all NSS-compatible browsers, but must be registered manually by and for each user. Instructions are included.
On my machine, this is consistently done by the "patching script interpreter paths" phase of the installation. Doing it manually seems like a step back, or at best duplicating effort.

Is relying on that unreliable/deprecated? Am I missing something obvious (chances are I am :-)?