Remove Ruby-based integration tests.
Added an initial integration test.
Started to work on logic to redraw viewmodels when the underlying model element changes.

Also, extracted increased commonality in Rectangle, RoundedRectangle and Ellipse into a Bounded superclass.
Implement redraw for remaining paths and stencils.
Implement redraw for changes to a label's placement.

As a consequence of adding this feature, the design of labelled paths and stencils was simplified a little. Previously a labelled stencil could return any kind of path when placement was sent to none. Now a labelled stencil always returns a labelled path, and the visibility of the underlying label is controlled by the placement property.
Added some discussion of the new viewmodel architecture.