Fix namespace usage such that all classes in shared/qml live in the
Qml namespace, and classes in QmlDesigner plugin in the QmlDesigner
namespace.

This should prevent any symbol clashes until we can finally get rid
of the file dublicates (e.g. QmlDocument).

 * Add a simple method to test whether a CMake target exists.

Reviewed-by: dt

This assumes that new Qt version entries will be put into
the System scope settings file.

This commit add a Settings::Scope parameter to
Core::ICore::settings(), the default is UserScope,
which falls back to SystemScope. Specifying SystemScope
instead gives exclusive access to the SystemScope.

Note that according to QSettings design, the file itself then is
in $CREATORDIR/share/qtcreator/Nokia/QtCreator.ini. Note case (Unix)
and Vendor dir!

* Add a new base class to derive build- and runconfigurations as
   well as buildsteps from.

Reviewed-by: dt

They do what you expect them to do, that is build only a subtree.
The implementation is a little bit strange. We temporarly set the sub
node to be built on the BuildConfiguration, enqueue that
BuildConfiguration (which calls BuildStep::init()), and then reset that
temporary value.

A more general way would be to have the ability to extend
BuildConfiguration::buildProject() in a way that additional data can be
passed to the BuildSteps.

Task-Nr: QTCREATOR-5
Task-Nr: QTCREATORBUG-44

Actually that's more correct. E.g. imagine building a project, and while
the qmake step runs changing the working directory. The make step would
be affected by that. With the change both the qmake and make step get
their working directory on beeing added to the queue and so don't care
if the setting is changed afterwards.

Also: re-enable 'Step over' fallback, report error for failed breakpoints

See BAUHAUS-277