Improve plotter new creation handling

Summary

Currently when creating a new Plotter, the prototype contains "plot/unknown/1".

This has problematic implications:

• The editor will fail to load it
• Test will fail because the editor can't load that dataformat

Steps to reproduce

1. Click on new
2. Select Plotter
3. Try to edit the plotter created

What is the current bug behavior?

An error is raised and it make the application fail

What is the expected correct behavior?

Either:

• Show a dialog asking to select a dataformat to use.
• Select a suitable dataformat from the prefix. Maybe a simple default like for the algorithm prototype.

Possible fixes

One possibility could be to have a method called after the dialog has been closed that can be re-implemented in sub-classes to do whatever is needed for the newly created object. Another would be to move the dialog logic in its own method so re-implementing _createNewAsset would be easier.

Note that this affects !101 (merged)