This control is almost entirely taken from http://www.codeproject.com/Articles/23746/TreeView-with-Columns by jkristia.

Minor changes have been made to clean up, rename or reorganise - as well as a few fixes to the non-visual styles rendering path
and some additional changes for my use of the control (like having the tree controls in a column other than the first, which mostly
'Just Worked' (tm) ).

So all credit goes to the above author!