nitpicker: Reworked session interface

This patch changes nitpicker's session interface to use session-local
view handles instead of view capabilities. This enables the batching
of multiple view operations into one atomic update.
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: rename apply_domain to apply_session_policy
nitpicker: Remove <global-keys> compound node
nitpicker: Hide global-operation key events
nitpicker: Add label arg to connection constructor
nitpicker: Draw black view if no buffer is present
nitpicker: Support for opaque views in X-ray mode

A session can be explicitly configured to present its views in a
completely opaque way when the X-ray mode is active as opposed to the
default where each view gets tinted and surrounded by a frame. This
is useful for decorator views, which look overly busy otherwise.
Adaptation to nitpicker config change
Add Rom_session::update

The new Rom_session::update function can be used to request the update of
an existing ROM dataspace. If the new data fits into the existing
dataspace, a subsequent call of 'dataspace' can be omitted. This way,
ROM dataspace updates don't suffer from page-fault-handling costs that
would occur when replacing the dataspace with each update.
Scout: Add DroidSans-Bold10 font as tff
Minimalistic decorator for window system
decorator stress test
Simple desktop environment (WiP)
Adapt qpluginwidget to nitpicker change (WiP)

Right now, it compiles but it does not work.
qt_desktop.diff