#22 Show scale on map

[stefan]

Show scale on map as known from Google Maps etc.
Allow the user to preselect if metric or imperial format is used.

[r_x]

A map ruler showing the map scale is a good idea. I just implemented a simple ruler showing the metric system. The imperial one comes the next days.

[r_x]

Ok, I learned that this tiny little thing is called "map scale bar".
Nevertheless SVN rev 505 contains the implementation of a scale bar that can be switched between metric/imperial format.