Open graph with an option to keep existing context

[paulgirard]

User story:
I worked on a graph and saved it.
I want to keep my app state (typically color palettes #61) but want to start with a blank graph to create it manually.
Thus I can open a blank graph but currently this button reset app state.
It should only reset graph and graph model.

To identify what state should persist in such a case we need to work on #34.

[paulgirard]

• open a graph with an existing context