Downstream users still import marker_trait from chaco.api

[aaronayres35]

This was removed in PR #679

To avoid breaking code we should bring it back into the chaco.api for the 5.0.0 release

[thomas-bersano]

I would like to note that the first example of a dynamically changing plot in the tutorial on enthought.com relies on marker_trait from chaco.api and therefore can not be run in the current version of chaco.

[gapster]

The tutorial on enthought.com is broken because you created a backward incompatibility. (Really? Who does that.) Please fix the incompatibility, or at least fix the tutorial. This will turn away new users.