Improvements to Measure.py extension

I made some improvements to the measure.py extension which can be used to
add measures to technical drawings.

I added input fields for
 * unit (m, cm, mm, in, px, pt)
 * precision (number of digits)
 * offset (distance to path)

I added precision rounding and fixed the text alignment to the middle of
the path.
The conversion of the length to a string is using the current locale now.

For me it is now a very helpfull tool, for others maybe too.

My suggestion is that this function should get a button in the toolbar.