[WIP] OWPythonScript: Rewrite

[irgolic]

Description of changes

The code editor (a QPlainTextEdit) is replaced with Qutepart (a QPlainTextEdit subclass). The source code for this was copied into Orange.
The console is replaced with a jupyter qtconsole, which runs a customized IPython kernel.

The main process and kernel process communicate in JSON, so variables are pickled, base64 encoded, sent, decoded, unpickled. This is very space inefficient. Perhaps something like https://jsonpickle.github.io/ could alleviate the issue?

The library stores files in a real folder, allowing users to import from their other scripts. Scripts aren't saved by default.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #4402 into master will increase coverage by 3.20%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #4402      +/-   ##
==========================================
+ Coverage   84.24%   87.45%   +3.20%     
==========================================
  Files         283      405     +122     
  Lines       57873    74084   +16211     
==========================================
+ Hits        48756    64787   +16031     
- Misses       9117     9297     +180     

[irgolic]

This is pretty much finished, just documentation left.

... Except, tests show [IPKernelApp] WARNING | Parent appears to have exited, shutting down.. I've not found a way to gracefully shut down IPYKernels (see OWPythonScript.onDeleteWidget), it seems that it's linked to a heartbeat pinging the parent process to check if it's exited. This isn't really a problem, except I've seen tests timeout sometimes on waiting for script execution, which might be related.
Although looking at ipykernel.tests.test_embed_kernel.py, they run some tests as @flaky(max_runs=3). If tests timing out remains a problem, this will most likely solve it.

@ales-erjavec, could you take a look to see if I didn't royally screw something up?
@ajdapretnar, I've fixed the comments from our discussion earlier today.
@janezd, could you play around with the widget a bit, tell me what you think about the UX?

Also, if anyone wants any specific keyboard shortcuts in the widget, let me know! The editor already supports stuff like moving lines (Alt+Up/Down), see the WIP documentation for more.

[irgolic]

I've gone ahead and added completions in the editor as well, modeled after spyder-ide. What's left is tooltips, like in the console, but I'm leaving that for some other time.

The only issue left is performance. The kernel starts up slowly; it takes at least a couple of seconds each time. Launching qtconsole as python -m qtconsole is instant, so I must be doing something wrong. Note, it exhibited this behavior from the very beginning. If this isn't solved it's not that big a deal, but if someone figures it out, I'd be immensely grateful.

[irgolic]

Closed in favor of biolab/orange3-prototypes#219.