Implement pure version of todos example 🎉

The `Widget` trait in `iced_pure` needed to change a bit to make the
implementation of `Element::map` possible.

Specifically, the `children` method has been split into `diff` and
`children_state`.

examples/pure/todos/README.md

@@ -0,0 +1,20 @@
+## Todos
+
+A todos tracker inspired by [TodoMVC]. It showcases dynamic layout, text input, checkboxes, scrollables, icons, and async actions! It automatically saves your tasks in the background, even if you did not finish typing them.
+
+All the example code is located in the __[`main`]__ file.
+
+<div align="center">
+  <a href="https://gfycat.com/littlesanehalicore">
+    <img src="https://thumbs.gfycat.com/LittleSaneHalicore-small.gif" height="400px">
+  </a>
+</div>
+
+You can run the native version with `cargo run`:
+```
+cargo run --package todos
+```
+We have not yet implemented a `LocalStorage` version of the auto-save feature. Therefore, it does not work on web _yet_!
+
+[`main`]: src/main.rs
+[TodoMVC]: http://todomvc.com/