used findDescendant to get the 1st element to open up in the content view
add depth first search to trees to find the first piece of content to open

also, set `_current*` to be the dereferenced Model because content may
appear in more than one book
change 'selected' attribute to '_selected' since it is a UI attribute

to match _isDirty and _hasRemoteChanges
Merge pull request #48 from oerpub/phil-selection-refactor

Phil selection refactor