Rdef slider range 8758 develop

This is in dev_4_4 as https://github.com/openmicroscopy/openmicroscopy/pull/1507, now for develop...

@chris-allan @cneves Sorry - should have cc'd you before on the dev_4_4 PR.
This makes a small change to the behaviour of the sliders when you manually enter window start/end values in the rdef Edit panel (enter values outside of 'min' and 'max').