Split canvas.py into subpackage

[HalfWhitt]

canvas.py is currently 1,747 lines long, and the actual Canvas widget doesn't start until 1,209. This PR splits it up into more manageably-sized files. The __init__.py exports all of the names, so nothing should change as far as use from outside of Canvas.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

We can remove the 2 "organisational" comments that you've flagged, but otherwise this looks like a good organizational cleanup to me.

On the subject of cleanups - if we're doing a big cleanup like this, then it might be worth finalizing the deprecations. The deprecation paths have been in place for over 18 months, before the 0.4.0 release; since we're about to bump to 0.5.0, we might as well finish that cleanup. I'll leave in your hands wWhether we clean up the canvas deprecations in this PR, or do a separate "Remove pre 0.4.0 deprecations" PR.

[HalfWhitt]

I'll leave in your hands whether we clean up the canvas deprecations in this PR, or do a separate "Remove pre 0.4.0 deprecations" PR.

Since it would be a good idea to clean up other deprecations too, yeah, I think I'll do that in a separate PR.

[HalfWhitt]

Question: should I also remove things that were marked as deprecated in the 0.4.0 release, or only those already marked before it?

[freakboy3742]

Question: should I also remove things that were marked as deprecated in the 0.4.0 release, or only those already marked before it?

If the source tarball for 0.4.0 contains the code for the deprecation, the deprecation can be removed for the 0.5.0 release (which will likely be cut once I'm back in the office in the new year).

[freakboy3742]

👍 Thanks for the cleanup!