Feature: Save the diagram on localStorage, add export/import diagram

[rubenmate]

Background:

The app state is volatile, when the page is refreshed it will start over.

Goal:

Implement a feature that saves the diagram to localhost so when the page is refreshed it will check if there is something there and load it.
This allows to import diagrams imported as JSON.

Proposed Solution:

Save the diagram structure in localhost. When the component is mounted check if there is a diagram saved and load it.
Add export/import buttons to export/import the diagram as JSON.

[rubenmate]

I have also added buttons to export/import the diagram as JSON.