Add settings backend and allow to select navigation buttons
Use Qt.labs.settings to store application settings and allow users to enable/disable showing of buttons on navigation bar. As a result, users can choose whatever they wish on the navigation bar individually. Current defaults are used as defaults for settings. In addition, webView settings are stored, as was wished in TODO.
I have kept window settings separate to for simplicity in definition of aliases.
Please review.
Fixes: #8 (closed)