Create a demo application

[Jacalz]

I think it would be a great idea to create a sort of demo application (in cmd/demo perhaps) to showcase most of, if not all, of the useful additions that this project provides over the regular fyne packages. Something like fyne_demo is what I am imagining. This is much better than adding a new project to the cmd/ folder for each widget.

This is not on top of my priority list at the moment so I'm opening it here so that anyone that wants to work on it can do so :)