GUI: wavelength can be set to negative

Instrument -> Beam parameters -> Wavelength can be freely scrolled and set to negative.

changed milestone to %21 GUI fixes

added GUI label

Handling of parameter constraints will change in core. We should discuss when I am back.

mentioned in issue #340 (closed)

mentioned in merge request !1047 (merged)

mentioned in commit b05c359b

closed with merge request !1047 (merged)

mentioned in issue #788 (closed)