Add comparison of blocks

When reading blocks, matching block entries are compared to see if the
contents differ.
Tree view output now displays "Matching" on those entries that contain
the same text.
Tree node tags now contain the original unique ID, whereas the string
could vary.
Add root nodes for matching and one file only

Adds root nodes for matching, and only in first/second file blocks.
The matching unique blocks are then sub-nodes to these.