Add display level enabling/disabling of the depth buffer (default remains: enabled).  This
allows 3D displays in 2D mode to have last DataReference added rendered on top w/o required
ScalarMaps to Display.ZAxis.  Useful in displays with line, point and non-filled geometry
when transparency is not required.  (2) Add some rendering attribute capabilities for more
flexibility.
setGeometryCapabilities -> ALLOW_REF_DATA_WRITE
add accessor for KeyboardBehavior
Merge remote-tracking branch 'upstream/master'
add a missing import
Merge pull request #21 from tomrink/master

Display level control for depth buffer enabling and other minor changes outilined in the commits