Expose scrolling api

[JordanMartinez]

Relates to FXMisc/RichTextFX#389

[JordanMartinez]

One caveat to this. I wonder if ScaledVirtualized should have two kinds of methods: one that exposes the regular API and a second that applies the scaling modification to the API. For example:

ScaledVirtualized sv = //
sv.getEstimatedScrollX(); // the regular non-scaled value
sv.getScaledEstimatedScrollX(); // the scaled value

[TomasMikula]

Included in the updated 0.6-SNAPSHOT.