Toggle button and sidebar visibility

[jtpio]

Originally posted in #239 (comment).

If we want to be able to manage the lifecycle of the debugger and the sidebar separately, we need to handle a few other cases:

• Automatically create the sidebar when switching the button on
• Closing the sidebar should switch the buttons off.

At first it sounds reasonable to always show the toggle buttons if the kernel supports debugging.

At the moment we can end up in situations like the following:

[jtpio]

An alternative could be to have something similar to the notebook tools panel in the left area:

The debugger sidebar would be added automatically if the kernel supports debugging, and removed when switching to a kernel without support for debugging, or to a different widget (not backed by a kernel).

But even that can lead to some tricky edge cases. For example the notebook tools in JupyterLab is not removed from the left area when the notebook widget is disposed but doesn't have the focus:

[jtpio]

Now that the debugger extension uses the kernel spec to know whether a kernel supports debugging: #457

One way to implement such UX would be:

• check all the available kernel specs
• if at least one kernel supports debugging, show the sidebar
• otherwise hide it

A couple more ideas were mentioned in jupyterlab/frontends-team-compass#75, so this is of course open for discussions.

[jtpio]

Looks like hiding the whole sidebar when no kernel with support for debugging (i.e xeus-python not installed), sound like the way to go for now.

This will make the debugger frontend completely invisible to end-users that install JupyterLab from scratch without xeus-python.