Create journal for graph when opened

Gives ability to undo/redo for all graph changes during a session.
No persistence for now.
elements: Add noflo-journal-history, with Undo/Redo buttons
Add hook undo/redo card to right sidebar

Known issue: undo/redo of node add/remove causes it to reset to 0,0
Merge pull request #93 from jonnor/add-undo-redo

Add undo redo