Enable resize of the tree view, using a paned view.
Fix #24: This seems to fix the problem with the window growing.
Fix #10: Collapsing tree view is no longer needed.
When a document is restarted, switch to the new tab automatically.
Remove unused argument.
Merge commit '28d09cbde721d63ead9ff3184827714117095ba6'
Avoid crash if no Document selected.