xf86-input-synaptics: make use of 50-synaptics.conf

The default synaptics functionality (without this file) is limited for
clickpads: the right soft button area in the bottom right isn't active by
default, so the entire pad generates left-clicks.  There is no way to
right-drag.

This file defines soft button areas and provides some matching rules.
These settings don't conflict with the synaptics options that NixOS
provides.