XML namespace

[NeilFraser]

Currently all XML nodes (block, shadow, field, mutation, variables, variable, comment, and probably a few others) are created with document.createElement (either explicitly, or via goog.dom.createDom).

This creates HTML elements, and such inherit a bunch of unexpected properties and methods, some with magical side-effects. The solution to this would be to use a namespace for our XML:
https://developer.mozilla.org/en-US/docs/Web/API/Document/createElementNS

To demonstrate:

var b = document.createElement('block'), i = 0; for (n in b) i++;
-> 233
var b = document.createElementNS('blockly', 'block'), i = 0; for (n in b) i++;
-> 137

And the above example is using a node name that doesn't exist in HTML ('block'). There are even more properties on nodes that do exist in HTML (e.g. 'img').

This should probably be investigated prior to work to remove goog.dom from the codebase (as part of the Closure removal process) and before we switch to passing around XML nodes rather than strings of XML (such as Blockly.Variables.generateVariableFieldXmlString).

Another factor that needs to be considered is Node.js' inability to call document.createElement or document.createElementNS since there's no document object. We need a cross-platform method of creating XML elements.

[jollytoad]

Selecting an alternative namespace would be highly desirable from our point of view - we are currently processing the blockly xml files in Java, and have to deliberately fiddle with the xml ns decl beforehand.

It would also be useful for other development processes, hand editing/viewing of the xml files - I work in IntelliJ IDEA which complains bitterly about the abuse of the html namespace ;)

We'd be able to provide a schema of some kind for XML too if able to set the namespace to help with manual viewing/editing, and allow validation against it.

I'd be happy to contribute towards this effort if it's deemed to be a direction you want to go.

[NeilFraser]

Fixed.