Enaml-web

- Allow `Raw` node to accept an etree element for the it's source.

- Move to python 3 only syntax and add typing annotations
- Reformat with black
- Add `dragstart` and `dragover` events to `Tag`
- Slightly speed up init_widget and use lru cache for determining attributes
- Use lru cache for proxy resolve class
- Remove unused init functions
- Remove logger and database from WebApplication, if you want those add them
in a subclass.
- Change id generator to use a shorter (but slightly slower) version

Make markdown source modified event return the rendered value

> This is the last version that works on python 2.7

Fix bug where Block with replace mode did not replace all child nodes

Allow passing keyword arguments to proxy.render and change encoding to 'unicode'
by default (tests show it's faster) and don't pretty print by default.

Remove `ref` and just use `id` instead