[WIP] IPyWidgets Backend for TraitsUI

[corranwebster]

Highly experimental, to the point that it possibly should live in its own project, but it can live here for now. To get it to work currently:

• create a new EDM python 3.6 environment
• install the ipywidgets with pip install ipywidgets
• install traitsui and dependencies with edm install traitsui
• remove the EDM installed traitsui with pip uninstall traitsui
• install this branch with pip install -e .

Things that still need to be done:

• build out a bunch of editors
• toolkit specific color and font traits
• image resources
• support for embedding in ipywidgets UIs (may work already, but untested)
• check various ways of configuring views
• clean up the ui_panel.py code
• had to remove StandardDialogMenu because we don't have menu support (and won't), so need to work out how to handle instantiating pyface menus, actions, etc.

[corranwebster]

Looks like test failure in last commit was because Github was inaccessible - re-started the failed test to be sure.

[prabhuramachandran]

@corranwebster -- thanks. there are a couple of things that you might want to refactor:

• In the last commit there is a _repr_html_ method that I inject to make it easy to display a UI, this really should go somewhere else, I was not sure where though.
• The button editor doesn't work because pyface doesn't implement
  resource_manager:PyfaceResourceFactory for the ipywidgets backend. I see that you already have this in the list of issues above but thought I'd just mention it here.

I'll try to push a few more editors as time permits.

[corranwebster]

It looks like IPyWidgets buttons are limited to showing icons from the Font Awesome icon set only, so we probably won't be able to integrate with that.

[prabhuramachandran]

Yes, it might be a good idea to take some notes on what we would need better support for and send that to the ipywidgets devs.