Move functionality out of NotebookPlugin

[jitseniesen]

The NotebookPlugin class integrates the plugin into Spyder. However, it implements more of the functionality, like managing notebook tabs and creating new notebooks. This should go in a different class to get a cleaner design and hopefully easier testing. Perhaps I can make a stand-alone application, which can be helpful during development.

[goanpeca]

The NotebookPlugin class integrates the plugin into Spyder. However, it implements more of the functionality, like managing notebook tabs and creating new notebooks.

Makes sense

Perhaps I can make a stand-alone application, which can be helpful during development.

Definitely useful!