[WIP] Updating (refactoring, 0.34)

[ellisonbg]

This is a start to refactoring the code base and adding support for JupyterLab 0.34.

[ellisonbg]

OK I almost have this compiling, but there is still a good amount of refactoring to do:

• [ ] Move all the toolbar creation function to a separate namespace+file.
• [ ] Use the toolbar attribute of the panel in the index to setup the toolbar buttons and get rid of the toolbar logic in the panel.
• [ ] Refactor the options/args to the constructor of VoyagerPanel, passing content: new Widget()
• [ ] Combine the VoyagerPanel and VoyagerPanel_DF into a single class with appropriate options.

Probably more to do beyond that though, but this is a start...

[timkpaine]

taking a look now