Ability to restore state

[DejanMilicic]

In the usual course of work, users might dive deep into various function definitions that will be opened in a main area.
If work is interrupted, the user will close the application.
When continuing work, the user will start the application again, but the application will start clean.

The ability to restore the state would be beneficial for the workflow.
Minimally, definitions in the main panel would be restored.
Ideally, the left pane tree would be restored, too.