Initialize tool with a relevant demo

[sjvrijn]

This request is inspired by the Case Law Exporer app.

By loading the tool with a relevant demo will invite new users to explore the tool and see what is possible, without needing to construct a dataset in the required format first.

This should only be loaded if no existing session is present, and should not introduce more clicks or actions to load own data.

[sverhoeven]

There is a load example button that loads a 6 statement and 3 connection example.

Also in #161 another example is added for the proposing of connections.