Mirror PythonFileEditor and Editor trackers

[karlaspuldaro]

Fixes #586
Fixes #520
Fixes #230
Fixes #127

Enables Create New Console for Editor from the context menu (right-click on the editor).
With the console running, auto-complete feature is accessible from the editor as below:

Enables TOC's python generator to work by default via the Editor tracker

Enables all FileEditor feature that are ties to it's tracker, including:

• themes
• search

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[karlaspuldaro]

After doing some investigation, I found that our python editor never sets its own session, which represents a live connection to a session kernel, basically a manager that owns the kernel.

In our current implementation, we create a python kernel and manually runs/stops it, only reading the input once at runtime, and exposing the output when it's done, there is never a live session running (removing the shutdown command after a run is not enough) - hence the option to create a python editor console is disabled from our extension.
Code completer needs to talk to the kernel through its session, so we need to set it up and expose it from the python runner.

[karlaspuldaro]

Instead of re-implementing the auto-complete on python file editor, the team agreed in enabling the existing 'Create console for editor'.
By manually adding create console command to python editor, as suggested, would end up with 2 commands in the context menu, one inherited from fileeditor-extension, and one from python editor.

The create console command was being disabled in python editor because the condition coming from fileeditor-extension was

const isEnabled = () => 
  tracker.currentWidget !== null && tracker.currentWidget === shell.currentWidget;

And it was found that tracker.currentWidget wasn't pointing to PythonFileEditor when it should. The solution was to add PythonFileEditor widget to an editor tracker.
This also solves the option to add a console from the main menu File --> New Console for Editor (before the changes, this option was disabled, with display New Console for 'Activity' )

[ajbozarth]

I've pushed an update removing redundant code that is no longer needed once we mirror the PythonEditor tracker with the Editor tracker. I've also updated the title and description for this PR to match the broader scope of this fix, as it now address multiple Issues.

[ajbozarth]

Including my updates I've tested this and it addresses all the missing features mentioned. LGTM

[ajbozarth]

copying my last commit message her for reference:

Updated to use inject

As per the doc and previous work by @karlaspuldaro we should use
`inject` rather than add. This change requires the addition of a
missing css selector for the context menu to still work though.

[karlaspuldaro]

Using inject instead of add breaks the python editor factory upon reload or changing tabs :(

[ajbozarth]

Upon further testing and research I've reverted my last commit that switched to inject. As @karlaspuldaro previously mentioned the FileEditor widget includes a check against tracker.currentWidget which is specifically not supported when using inject. I am unclear if this is a mistake on JupyterLab's part or if this is just not actually a use case for inject. I am worried that by using add instead of inject we may run into issues in the future due to two trackers tracking the state of a PythonEditor, as of now it doesn't seem to be causing any problems. So this is still LGTM

[lresende]

Applying "Jupyterlab 1.x" as this needs backport to 1.x branch. I have started the backport but needs a little more work as TOC stopped working (we might need a smaller cleanup on 1.x than in 2.x).