Develop a GUI functional testing crate

[antoyo]

• Fix the relm-test crate.
• Create an Observer astraction.
• Add useful functions in it.
• • Click any widget (on the center of the widget or at a specific position — test that it works for menus).
• • Double-click.
• • Mouse move.
• • Mouse press and release.
• • Send input (key press) to any widget (one key and a string).
• • Send key press and release.
• • Wait a specific amount of time.
• Write an example like the one on this article.
• Add example of clicking/entering text in a relm widget.
• Allow to act on widgets contained inside relm widgets (like entering text in a child widget of a relm widget).
• Test all the examples.

[antoyo]

Having some basic crate to do functional testing and an example of using it is a must for 1.0.
PR #100 is a step in this direction.