Add a button to open and close, and keep close the proof view if used

[thomas-lamiaux]

There is currently a button to close the proof view but it reopens constantly at each interaction which is not always wanted as some users:

• don't need it for what they are doing, e.g. meta-programming
• split their screen

Feature request: turn the close button in a open / close button, and if the window has been shut then it is not reopened unless the user presses the button