Data binding

[Berrysoft]

We need to provide a convenience way to bind the data.

Consider a type Property<T>, and make some properties like

pub fn text(&self) -> Result<Property<String>>;

pub fn set_text(&mut self, s: impl Into<Property<String>>) -> Result<()>;

and users could write

bar.set_text(foo.text()?)?;

The widget bar saves the Property and starts it in start(). When the property changes, foo posts a message to the Property, so bar could receive the event.

cc @mokurin000 for advice