Created TreeAPI class, mostly for documentation.  This revealed one or two unused methods in the "real" leoTree class.  All tests pass.

Leo build: 20140908141111
Updated to-do list and removed unused code from the attic.

Leo build: 20140908141310
Modified a unit test so it works regardless of whether Qsci can be imported.

Leo build: 20140908141442
Changed the comment in the headlines for set_signals.

Leo build: 20140908141532
Merged BaseNativeTreeWidget into LeoQtTree.  There is no reason for this two-level scheme! All tests pass.

Leo build: 20140908141631
Completed cleanup of LeoQtTree class.  Just reorganized nodes and simplified headlines.  All tests pass.

Leo build: 20140908143231