Implement property to property mapping

[SamRodri]

In widget macros, implement mapping a property value to another property in the same widget using the expr_var syntax.

#[widget($crate::foo)]
pub mod foo {
    properties! {
        a_property;

        /// This property is set only when `a_property` has a value and it is a mapping of the a_property.
        b_property = 1 + #a_property + 3;

        /// This property is set only when `a_property` is and it is a mapping of the a_property.
        on_prop = hn!(#a_property, |_| {
            println!(a_property.get());
        });
    }
}

• How does this integrate with when?
  • Not allowed in when assigns?
  • In the example above a_property is affected by when in the handler.
• Use cases:
  • Border color depend on background color with when only assigning background.