os: forward args for Lazy_volatile_object

This patch makes the handling of constructor arguments consistent among
the Volatile_object and Lazy_volatile_object classes. Arguments are
always forwarded. Otherwise, passing a reference as argument would result
in an unwanted copy of the passed object.
nitpicker: Domain configuration

This patch introduces the notion of a "domain" to the nitpicker
configuration concept. Session policies always refer to a domain where
multiple session policies can refer to the same domain. Thereby a domain
provides a way to express the grouping of sessions. This is useful for
applications that open multiple nitpicker sessions (such as Qt5 apps that
use one nitpicker session per window, menu, etc.). We want to assign all
those sessions to a single domain.

The configuration looks as follows:

<config>
  ...
  <domain name="default" color="#ffffff"/>
  <policy label="" domain="default"/>
  ...
</config>
nitpicker: Keep focus when activating xray mode