Also, the Tree object was only ever used by the TreeMapWidget, so its
creation can happen inside `analyze()`, fixing the memory leak issue.
Plus, it doesn't have to be RefCounted.
Made the member fields of Tree and TreeNode structures private and moved
the functions in main.cpp that accessed the internals of these
structures inside the TreeNode class
The TreeMapNode and TreeMap structs inside TreeMapWidget.h both had
single implementers, TreeNode and Tree inside main.cpp. The indirection
was removed and the new structures were moved to their own file
