Adjust names of Canvas draw handler interface and view

[paulproteus]

Rename CustomView to DrawHandlerView

Rename IView to IDrawHandler

I believe this makes the purpose drastically easier to understand. See https://github.com/beeware/toga/pull/1309 for discussion.

PR Checklist:

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