Custom Element rewrite

[matt-gadd]

Type: feature

Description:
This is a work in progress complete rewrite of our Custom Elements implementation, mainly to support #809, and in general remove a lot of old cruft. I will add more detail to this description and to the readme, but a quick list of what has changed:

• only 1 module to actually support custom elements (rather than being spread across 2)
• removed support for descriptors entirely (the decorator is now the only mechanism that can be used)
• simplified the format that the decorator puts on to the widget proto
• removed a lot of the functionals in favour of units
• supports children and properties on children (introspection and writing from parent like: Custom elements do not have access to the properties of their children #809)
• can add children post appending the parent
• supports future functionality for custom elements using a global theme

Currently does not support:

• an initialisation function in the decorator
• any kind of attribute transform, attributes can only be string types