Add link to JFX-Central in help menu

[Oliver-Loeffler]

As discussed in #484, it can be beneficial to have a link to https://www.jfx-central.com.

Expected Behavior

The help menu already collects links to JavaFX related documentation. Here seems to be a good place to also hava a reference to https://www.jfx-central.com.

Current Behavior

There is no reference to https://www.jfx-central.com in Scene Builder.

Context

Scene Builder is often an entry point to the JavaFX world and JFX-Central has grown to a size where it is a very useful resource for a JavaFX developer.