Clicking 'Change Kernel' for interactive window started from script file does nothing

[joyceerhl]

1. Have an active interpreter without ipykernel installed
2. Start the interactive window from a script file
3. Click 'Change Kernel' on the dialog that comes up
4. 🐛 Nothing happens. This is because notebook.selectKernel doesn't work if there isn't an active notebook editor

Based on discussion at notebook sync this morning, we can modify notebook.selectKernel to take the notebook widget that the kernel picker should be invoked for (interactive.open returns the notebook editor widget).