WIP: Started tidying up the tree browser
Finished initial tidy-up of new tree browser
Minor tidy before refactoring view mode MVC
Started refactoring view mode MVC slightly

I need to put Aardvark to bed now due to another interrupt. Note that
there's a bug in the GUI model when closing tree branches while viewing
a descendant node. I need to separate out the metadata for the
'selected' node from the tree model to stop this happening.