Corrected the value range for min and max length.

This fixes the problem when updating the filters: since the
min and max values were just cast to int instead of being
floored and ceiled, a small subset of fibers could be lost because
their length was higher than the max, cast as an int.
Added constants to initialize subsampling sliders.

This allows use to customize the Fibernav for people with
less powerful machines. We could for example set the start
value to 50, to already subsample on application startup.

This is part of ideas I have to allow easier customization, and
make the Fibernav work on more machines.
Fixed a small Linux bug for sizing of the slider.
Merge pull request #164 from jchoude/bf-37-subsamplingResume

Fix subsampling resume bug.

Closes issue #37.