Make the Uiml.net renderer easy embeddable in other applications

A third-party developer should be able to easily integrate the Uiml.net renderer in its application. Currently, to do this he needs 5 to 6 lines of code, exposing the inner details of creating a UimlDocument, using its vocabulary as input to the BackendFactory, creating an instance and showing this instance. We need one function that accepts a file name (or a string) containing UIML code, and which returns the resulting rendered instance, which can be shown or integrated inside the third party application. Of course the rendered instance that is returned, depends on the used backend (e.g. Gtk# or System.Windows.Forms).