icon sources
more specialised images for ProofParallel
upped the model numbers
created ProofId element to replace ProofParallel.links
extension point to add custom model evolution rules
fixed model references (part of version bump)
fixed adapter factory versions (part of the version bump)
bad copy-paste (forgot to change typecast)
display the ref element for ProofId in the tree
fixed DB upgrade: now resolving cross-references correctly
upgrade ProofParallel.links data to ProofIds
replaced parallel soft-links with ProofId in PPTree-Graph conversion
additional tests
test graphs values are method-local, not global
adjusted formatting; renamed m/s/p variables to better names

Note: some tests are failing because the graph has rearranged?
use Set for PPGraph roots (partial change)
reworked PPGraph <-> PPTree conversion

Now creating PPTrees with better merges. Also should be handling all
graph cases by using ProofId references.
cleanup
reorder code
cleanup