Move the dash.layout.setter so that type checkers can find it easier

[peter-gerlagh-kyos]

This rearranges two methods of the Dash class.
Specifically you now have @property def layout // @layout.setter def layout // def _layout_value instead of @property def layout // def _layout_value // @layout.setter def layout.
The point is to make it easier for type checkers (such as mypy) to connect the @property layout with the layout.setter, so that people who use this library with a type checker need fewer #type: ignore statements.
This is a very minimal change, and I don't see how it could break anything, or make anything worse. I don't think a note in the changelog is necessary.

Contributor Checklist

• I have broken down my PR scope into the following TODO tasks
  • change order of methods
  • confirm mypy is happy
  • run tests
• I have run the tests locally and they passed. (refer to testing section in contributing)
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

optionals

• I have added entry in the CHANGELOG.md
• If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLS as follows
  • this GitHub #PR number updates the dash docs
  • here is the show and tell thread in Plotly Dash community

[peter-gerlagh-kyos]

I've not been able to run the test, and I don't understand the problem. dash_test_components seems to be missing (?)

[T4rk1n]

I've not been able to run the test, and I don't understand the problem. dash_test_components seems to be missing (?)

For the you need to run npm run setup-tests.py, but it might takes pretty long, can just run the tests that fails the ci.

[T4rk1n]

Look good, if that helps with linting doesn't hurt to change the order. 💃