Use relative paths to specify layout & test repo

Use OS-agnostic path joining function to specify the mainUI file
and the test repo folder as relative to the project root so the
program will execute on all machines with proper folder structure
Connect git api to canvas to show test repo graph

Attach a GGraphicsScene to our UI and give it the root commit
returned by the git parser.

Fix up nodes to show 6 digits of the sha whose commit they represent
Merge branch 'master' of github.com:AnthonyReid99/VisualGit