NetBeans 8 port of JavaFX based preview

This is the same as my previous experimental version of the JavaFX based preview. Thanks to new features in NetBeans IDE 8 it no longer requires custom extensions to the build script or class loader magic. Due to the same reason it also should no longer conflict with other JavaFX enabled plugins.

**Requirements**
This version of the plugin requires NetBeans IDE 8 and JDK 7 with bundled JavaFX 2.2 or JDK 8. Strictly speaking it should work with a JRE but (parts of) the IDE itself require a JDK.

I have uploaded a [pre-release][] for evaluation.

[pre-release]: https://github.com/stengerh/flow-netbeans-markdown/releases/tag/v1.6.0-experimental