Added a set of radio buttons and code for controlling history traversal method, either shallow or deep. Worked on lower panel alignment.
Added first attempt at overriding collapse behavior for TreeView.
Fixed TreeView subclassing to disallow collapsing. Changed history labels layout to gridlayout to fix alignment problem. Thanks @quanon!
Changed lower panel to use GridLayout, and added column labels.
Formatted long strings for pep8.
Combined process A and B go_to_history_state() buttons into one grid. Working on other widget layout problems.
Merge branch 'master' into develop
Changed the top toolbar to use toggle buttons, and to be 30px high.