Persistent graph view

As suggested in #40 I implemented persistent positioning for graph nodes. Heads-Up: This is the first thing I ever did with rust, so it's hard for me to judge my codes quality. (Primarily I did this to get my personal version to a workable state)

A few more remarks / things to keep in mind for the future:

• Yes, this is a big change.
  • I'm not mad if this does not get accepted as-is ;)
  • If you don't want to include it like this, feel free to give feedback or just use it as (working) reference for your own implementation
• It might make sense to make this behavior optional e.g.:
  • Add a configuration section (or even just right-click context menu) with a toggle to save/restore node positions
  • Open a one-time dialog when first manually moving a node asking whether node positions should be permanent or not (could be reset by deleting the configuration file for the start)
• The relatively unique "ident" already works quite good, but is certainly not perfect
  • Add auto-incrementing suffix if multiple nodes with the same ident appear (done and working quite well)