This change moves the QtQuick 2 types and C++ API (including
SceneGraph) to a new module (AKA library), QtQuick.

99% of this change is moving files from src/declarative to
src/quick, and from tests/auto/declarative to
tests/auto/qtquick2.

The loading of QtQuick 2 ("import QtQuick 2.0") is now delegated to
a plugin, src/imports/qtquick2, just like it's done for QtQuick 1.

All tools, examples, and tests that use QtQuick C++ API have gotten
"QT += quick" or "QT += quick-private" added to their .pro file.

A few additional internal QtDeclarative classes had to be exported
(via Q_DECLARATIVE_PRIVATE_EXPORT) since they're needed by the
QtQuick 2 implementation.

The old header locations (e.g. QtDeclarative/qquickitem.h) will
still be supported for some time, but will produce compile-time
warnings. (To avoid the QtQuick implementation using the
compatibility headers (since QtDeclarative's includepath comes
first), a few include statements were modified, e.g. from
"#include <qsgnode...