Prefixed the framework API with $

There are two things impacted: 

- the implementation
- the code using the framework

Here are the element which have been prefixed:

- `attributes` property of components' controllers
- `on<Attribute>Change` change handler of components' controllers
- the `elements` property of components definitions, to define what kind of content they can expect
- the associated `content` attribute on components instances referring to the instances of these elements
- `event` object available in environment of event handler expressions
- `scope` object available in environment of templates

## Implementation

This includes:

- its code
- its documentation
- its tests

## User code

### Playground

This includes:

- the playground runner
- the samples
	- code
	- documentation