os: Add Input::Event_queue::reset function

Resetting the input-event queue is useful for error handling.
Add 'Weak_ptr' to the public Genode API

So far, the lifetime-management utilities 'Weak_ptr' and 'Locked_ptr'
had been preserved for core-internal use only. However, the utilities
are handy for many use cases outside of core where object lifetimes
must be managed. So we promote them to the public API.
nitpicker: Perform redraw asynchronously

This patch changes nitpicker's way of redrawing. Originally, redraw
operations were triggered immediately by the RPC functions invoked by
clients. In the presence of clients that invoked a large number of those
functions, the server could become overloaded with processing redraw
operations. The new version performs redraw operations out of band with
the RPC functions. Similar to the design of the DOpE GUI server, redraw
operations are processed periodically. The RPC functions merely modify
meta data and track the dirty areas that need to be updated.
Consequently, nitpicker's RPC functions become light-weight operations.

As a nice collateral effect of this patch, nitpicker's internal
structure could be simplified because the drawing backend is no longer
needed by the code that dispatches the RPC interface.
nitpicker: Support for session-focus management

This patch introduces a focus-management facility to the nitpicker
session interface. As a side effect of this change, we remove the notion
of a "focused view". There can only be a "focused session". This makes
sense because input is directed to sessions, not views.

Issue #1168
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.
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