Ignore drags if can't get local instance data.
Handle drag and drop on trees with hidden roots more gracefully.
Fix issue with inserts not working correctly due to incorrect calculations.
General fix-up of drag-and-drop logic.
Fix missing self in restore method.
Formatting of code.
Remove tab character, unnecessary whitespace.
Fix `if...elif...else` chain.
Merge pull request #126 from enthought/fix/qt4-tree-editor-drop

Qt4 TreeEditor Drag and Drop Improvements