propagate shading styles for newly created feature types to other alignment views

When a new feature type is created, the feature renderer for the view receives the new type's display style from the 'create features' dialog box. Other views and alignments, however, do not receive the new style - and will only display the new feature type after the Feature Settings dialog box is opened and a new colour is autogenerated.

Test this by:
1. create two views.
2. in one view, create a new feature with a novel type string.
3. in the other, open the feature settings dialog box - the new feature will be made visible, but will have an autogenerated feature colour.

Ideal behaviour:
Creation of a new feature type rendering style should result in the same style being used in all views concerning that dataset, but the new feature should not be made visible by default (although this might be considered a configurable option).

To achieve this behaviour, we could:
* broadcast a 'feature style update' message for the dataset so other views on this and other alignments can add the type and display style to their feature settings (and also decide if the new type should become visible).
* introduce a 'master' feature display settings that can be updated with the new type, so new views inherit the display style when their sequence feature settings dialog updates.