Cannot change a Page name after instantiation

[kharybdys]

I wanted to set the page name, and right now TreeDiagram instantiates its Page without name so it's a change after the fact.

I can set page.name but it doesn't reflect in the generated drawio, because it actually needs to be set in the diagram.

The logical thing is to create a property with setter that reflects the diagram, similar to a lot of other properties around the library.

I have a branch already with the fix + new test.