UI support for floating point channel values

Changed the channel slider to support floating point values; also changed the layout of the metadata panel a bit, so that extra space can now be used by it's sub-components (e. g. the channel sliders).

Test:
Modify the channel ranges via the sliders and the text boxes, using images with different floating point ranges. Also check that these operations are still working with integer type images. Check other parts of Insight, which make use of the slider component (e. g. the movie player dialog), and make sure that they still work like before. 

@jburel : I changed the TwoKnobSlider to work directly with double values, and therefore removed the 'rounding factor'; this should be ok, because no int conversion is necessary, or do I miss something here?