Multithread example

[grelltrier]

I think this is a better example of multi-threading because the user can see the other thread changing the counter and the new thread not instantly finishing.
I don't know how to write a test for this though

[antoyo]

Thanks for this PR!

Could you please post this as a separate example so that we'll have one multithread example using the #[widget] attribute and one without?

Can you try to write the test by taking as example the multithread tests?
That is, using relm_observer_wait!(let Increment(value) = observer); a couple times and asserting the value.

[grelltrier]

I can try but it will take some time. I am a bit overwhelmed with work right not :S

[micwoj92]

This branch has conflicts that must be resolved