This layers things properly to be able to implement the UI in the
example application instead of directly in shell_qt.cpp.

This is still using global variables to allow the Shell platform
code to do callbacks to the API classes. This should go away once
we properly implemented a WebContentsDelegate.

The current preliminary implementation uses the QQuickPaintedItem.

The RasterWindow is being replaced by an abstract NativeViewQt
class, which can be instantiated as QWidgetNativeView or QQuickNativeView.
The NativeViewContainerQt builds a wrapper around an instance of these classes
and serves as a common api towards chromium.

Due to the current design where the view is being created by the shell,
we introduce a browser_window.qml which provides a very basic browser UI.
The content is then being injected into an item within that browser window.

Just executing the example the "regular" way will launch the Widgets example.
To launch the QtQuick2 example, the environment variable QQUICKWEBENGINE must
be defined.