Convert to new pattern for mixin/traits

[zepumph]

From #143, here is the list of mixins and traits in the project that we should update to use the "new" pattern. See Popupable as an example of the new, es6 style of mixin/trait creation.

using mixInto: as a search

• Paintable.js
• Poolable.js
• Leaf.js
• AccessibleNumberSpinner.js
• AccessibleSlider.js
• AccessibleValueHandler.js
• ValueGestureControl.js

using "compose" as a search

• ParallelDOM.js

Marking for dev meeting as a chip away.

[pixelzoom]

3/4/21 dev meeting.

We discussed concerns about mixins. @samreid mentioned lack of IDE support, which is the case for the new and old patterns.

@samreid suggested that we investigate using compostion instead of mixins. Here's an article that enumerates pitfalls of mixin, and discusses moving from mixins to "automated composition":
https://raganwald.com/2016/07/20/prefer-composition-to-inheritance.html

We ran out of time, and decided to resume discussiion of this issue later.

[jessegreenberg]

Discussed during 3/4/21 meeting -

JO: There is an issue with the new pattern for mixin/trait where there isn't IDE support.
SR: Maybe we should transtion away from mixins?
MK: If IDE support is the only bad part of this, can we make our own pluggin to support it?

Sounds like there are loose ends still and we aren't ready to move forward with the new pattern yet. We will pick it up next time.

[zepumph]

Looks like we are not in the "now we convert" phase here, I will note that perhaps #143 is the better issue to discuss this in. Unassigning.

[zepumph]

@pixelzoom had the right idea over in #143 (comment)! Unmarking for dev meeting. On hold until we finish discussion over there.