use dependency state in export

[knaaptime]

resolves #726 again

[kylebarron]

It looks like this is documented here:

In all embedding functions in ipywidgets.embed, the state of all widgets known to the widget manager is included by default. You can alternatively pass a reduced state to use instead. This can be particularly relevant if you have many independent widgets with a large state, but only want to include the relevant ones in your export. To include only the state of specific views and their dependencies, use the function dependency_state

[kylebarron]

Thanks!