API design for choosing between multiple backends

[pllim]

Let's say in the future, we have astrowidgets API implemented with different visualization tools (backends); e.g., Ginga, glue-jupyter, PyWWT, etc. How do we enable switching backends?

• Do we do this at import time and then have it fixed for a session? Example: from astrowidgets.ginga import ImageWidget
• Do we allow user to switch inside an active session? Example: import astrowidgets; astrowidgets.set_backend('ginga'); from astrowidgets import ImageWidgets
• Something else I haven't thought of?

Also see: #93

[pllim]

With #162 , backend implementation would be completely separate and eventually they won't even be in this repo.