Clarify / Remove use of metadata, metadata_changed, etc traits

**Problem Description**
Currently Chaco does some pretty confusing things in regards to metadata.
There are:
- traits named metadata
- traits name metadata_changed
- magically named `_metadata_changed` methods which simply set the `metadata_changed` trait to `True` when the `metadata` trait changes
- a method name `metadata_changed` (no leading underscore) that gets hooked up as a change handler listening for changes on the `metadata_changed` trait. (Note this method has been renamed to `metadata_updated`)

All of this is rather confusing, and likely unnecessary.
We need to investigate how all of this is working more closely and see if it can all be replaced with newer traits features.