Allow the creation of namespaces higher in the DOM tree about dart-xml

[renggli]

The described solution (1) would be tricky to implement, but possible from within the XmlBuilder during the building of the tree. Unfortunately I am quite busy right now, so I don't have the time to look into it. However, free to submit a pull-request.

Alternatively (and if you don't want to modify the library) you could write a visitor over the built XML tree that transforms it into an optimised tree. Implementation wise I think this solution would even be simpler, albeit less efficient as the tree needs to be re-traversed and copied.

[stevenroose]

I will try to give it a look this week, time isn't my best friend either.

[stevenroose]

I got both 1 and 2 implemented. However, I want to know how you want users to opt-in or opt-out for the enhancements.

I'll make a PR and ask you there.

[renggli]

This is fixed with aebf5bd.