CQGI: require explicit cadquery and show_object imports

**Issue by [dcowden](https://github.com/dcowden)**
_Monday May 21, 2018 at 21:09 GMT_
_Originally opened as https://github.com/dcowden/cadquery/issues/270_

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after discussions with @fragmuffin and others, we'd like to change cqgi to require scripts to import show_object, debug, and cadquery itself.

The consensus was that there is just too much magic in scripts that use show_object when it appears out of nowhere.

This will require changing cqgi itself, as well as changing all of the examples to do this import. It probably also means that cqgi.show_object should be promoted to a real function, not injected during the ast phase.

@jmwright before i start these changes, do you agree? What is the impact on cqfm?