Add preview as you type for split view

You can now choose "Split Horizontally/Vertically" from the menu and use a preview-as-you-type mode.

Relative images and includes are now also shown in the preview. And it doesn't jump at end of preview on view refresh any more.

Tested locally with IntelliJ IDEA 14 & Java 8. Please have a look & comment or merge.

![preview_splitpane](https://cloud.githubusercontent.com/assets/3957921/5157565/46a80918-7314-11e4-9382-0da9f1d3660e.png)