GraphNode.setNodeStyle() doesn't work for ZestStyles.NODES_HIDE_TEXT

[ptziegler]

The style of a Zest graph node can either be set via the constructor or the setNodeStyle method. Similar to #639, I think it's a good practice to only use the setNodeStyle method, to have a clear separation between Zest and SWT styles.

Sadly, this doesn't work for all Zest styles, as e.g. the NODES_HIDE_TEXT only works when used within the constructor. This is because the underlying figure is initialized together with the graph node and never updated in case the style has changed.

Note: I don't want to prohibit the use of Zest styles in the constructor, though I would like to update our examples to use the latter as the "proper" way to use them.