add get_all_items, Draggable, DraggableSet
Fix for gighest target z-index.
Fixed IE10 bug. Now can drag without starnge behavior across all browsers.
refactoring: `on_target()` -> `on()`
Cherry picked commit abc8555f65d6f89d72761220c55eef63df74ae51.
Documenting ^_^v.
chem mode in DnD: separate labels in slider and icons in slider of same height are fixed
mathjax for label end: Mathjax labels are added
add "auto_resize" and "separate_labels" options for DnD, make lines in DnD prettier
chem DnD: Chem flags were partly implemented
Support new attributes for chemical problems. JS part is implemented
add DnD constraints documentation
use lower case for bool value in DragAndDropInput attrs
pep8 formating
DnD: add additional constraint `contains()` + test
DnD refactoring: python part
DnD refactoring: javascript part.
move draganddrop code to new package "draganddrop"
add DragAndDropResponse
put serialize `xml` to the python sandbox (context)