Add data bindings

[AaronErhardt]

Summary

Fixes #377

This WIP and currently meant as RFC.

This PR adds support for data bindings in the form of special GObjects that just store one value.
The interesting part is, that these objects have - similar to factories - a guard() method that creates an RAII guard.
This guard implements Deref and DerefMut for the actual data type you want to access, which makes it more convenient than using getter and setter methods.

For objects and widgets with a property that describes their state it is made even easier to connect a binding.
For example, ToggleButton has an "active" bool property that indicates whether the button is active or not.
This will most likely be the information you want, so a special trait provides the with_binding() constructor method to directly bind the state to the binding.

Check out the data_binding example to get a better idea of what I'm talking about :)

CC @Ayush1325

Checklist

• cargo fmt
• cargo clippy
• cargo test
• updated CHANGES.md