Move treeItem decorators  handling methods from SWT to JFace plugin

SWT TreeItem implementation is handling tree item decorations such a Git or Server decorations but in Eclipse these decoration are handled by JFace layer and RedDeer should be aligned with Eclipse architecture.

At the moment these methods should be moved from SWT AbstractTreeItem to RedDeer JFace plugin:
- parseText()
- getStyleRanges()
- getNonStyledText()
- getStyledTexts()