Support persistent 'perspectives' to handle arrangement of views/settings

angr-management will always start with a default arrangement of views + many settings. If you close a view and restart angr-management, the default arrangement will be displayed again. This can be annoying for users who prefer to have their interface configured in a specific way.

At the very least, angr-management should support recording open views and their configuration and restore them upon re-opening.

Expanding on this, some tools like Eclipse have a concept of 'perspectives' which records the arrangement of views. A user can switch between perspectives depending on the task at hand. I think a similar model could be a helpful addition to angr-management.