added "Zoom" button and improved scaling

It should be possible to have a certain limit where we
stop zooming so that short dives are visible as such
at first glance. Therefore a "Zoom" button has been
added to the "Log" menu along with a shortcut (Ctrl + "0").
The user can now zoom/unzoom the plot and is still able to
quickly distinguish short dives from normal ones when
browsing the log.

Signed-off-by: Maximilian Güntner <maximilian.guentner@gmail.com>