Add stop_tracking button to header bar

Created by: GeraldJansen

This PR adds a stop_tracking button to the header bar of the overview window (#258 (closed)). It complements the Ctrl-Space keyboard shortcut and does the same thing.

A few other tweaks are proposed: 1) the settings icon is replaced by an open-menu icon, 2) the order of the buttons is reversed to reflect current trends and to be more similar to apps such as yelp.