Create "gallery" showcasing available demos and sample code

[jpsacha]

ScalaFX code contains a good amount of various demos. Many users may not be aware of what is available. It will be great to have a "gallery" that will showcase the demos with screenshots. This is in addition to ScalaFX Ensemble. It would focus on code in scalafx-demos

Suggestions are welcomed on how to do it best in a form that easily accessible by users but also easy to maintain.

One possibility is to use the Wiki and manually add links and screenshots to the demos with a brief description what the demo illustrates.