Deprecate metadata_changed event

[jvkersch]

With the merge of https://github.com/enthought/chaco/pull/395, ScatterInspector.metadata_changed is redundant, and ScatterInspector.inspector_event should be used instead. This issue is to document:

• [ ] Switching over uses in the codebase of the former to the latter, and putting some more warnings in place that metadata_changed is redundant.
• [ ] Removing the actual event after a couple of releases have gone by.