fixes #LIBS-50
Adding a child to a Sprite removes its Graphics
http://jangaroo.myjetbrains.com/youtrack/issue/LIBS-50

A bit refactoring revealed and fixed the bug: after graphics
canvas was added, DOC replaced all DOM child elements
with the ones generated from its children, and Sprite failed
to re-add the canvas corresponding to its graphics when
the graphics were not dirty.
Merge branch 'jangaroo-1'