Add `Properties[Mut]` argument to widget methods.
Add a third TreeArena to sort per-widget arbitrary property values.
For the type-to-value map, I considered the following crates:
- https://docs.rs/typemap/latest/typemap/
- https://crates.io/crates/typemap_rev
- https://crates.io/crates/typemap-ors
- https://github.com/chris-morgan/anymap
- https://github.com/reivilibre/anymap3
Of these, anymap3 is the only one actively maintained (last commit less
than 12 months ago). The code source itself is extremely light and
simple; we may or may not decide to roll out our own implementation down
the line.
Add a BackgroundBrush property used by SizedBox as a proof of concept.
Note that SizedBox still has its `background` field, but we should
expect future widgets to use almost *exclusively* properties; properties
usually shouldn't be redundant with local fields.
To get there, we'll first need to better integrate properties in Xilem.
Olivier FAURE