Sidebar DnD is enabled on the wrong element

It must be an element with the data-media-type attribute for dropzones
to work properly
Getting children of a Tree Node uses getChildren

instead of assuming a Backbone attribute named 'contents'
Enable dragging only on the current node

not all of its descendants
Merge pull request #72 from Connexions/toc-getchildren

Getting children of a Tree Node uses getChildren
Merge pull request #71 from Connexions/hotfix-dragging

Sidebar DnD is enabled on the wrong element
Merge remote-tracking branch 'upstream/master' into regressions