Use api modules for ETS packages where possible

[rahulporuri]

fixes #618 and #605

This PR updates the code (package and examples) to make use of the api modules where possible for ETS packages.

[aaronayres35]

All the changes here LGTM

On this branch I am still seeing some imports that aren't from api modules though.

eg in chaco/api.py we import BaseTool and KeySpec from enable.base_tool not enable.api, or markers in chaco/scatter_markers.py, etc.

[rahulporuri]

in chaco/api.py we import BaseTool and KeySpec from enable.base_tool not enable.api,

I had opened #628

or markers in chaco/scatter_markers.py

Ahh. I forgot to open this issue. Looks like this module can/should be removed too - because code from here got moved to enable - and this module was kept around for backwards compatibility - https://github.com/enthought/chaco/blob/f7cdff7bac428852b343f5311fb397c08a19c789/chaco/scatter_markers.py. I'll open an issue

[rahulporuri]

I'll open an issue

added comment in #580