Tree plotting - linearise the plot and colouring algorithms

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The tree plotting and colouring algorithms use a recursive strategy.  The issue 
with this is that large trees will incur deep recursion and any associated 
overheads.  

Linearising the algorithm will (hopefully) speed things up, as well as make it 
easier to add a progress dialogue to the plotting process.

Original issue reported on code.google.com by shawnlaffan on 29 Jul 2014 at 11:21