Added drag and drop widget
Docs about drag and drop
DragAndDrop widget test
Swing-based expression editor
* main class is the ExpressionEditor, in editor core tests
* supports both graphical editing (in tree) and textual editing (in textarea)
* required minor changes to Literals and VariableRef expression nodes
Some refactor in sides layouts
Added a horizontal layout
Added an editable label
Added a scene tree widget
Javadoc
Updated API
Added IconButton widget
Added separator widget
Updated skin
Added test for IconButton and Separator widgets
Merge pull request #388 from anserran/hierarchy-widget

New scene hierarchy widget
Merge pull request #390 from anserran/simplebutton

Added IconButton and Separator widgets
Fixing master
Merge pull request #392 from jtorrente/fix-master

Fixing master
Some fix in docs. Remvoed unnecessary parameter.
Added test for modifier