gui: Bandwidth slider disabled colour fix and Slider refactoring to resemble the wx.Slider API more closely
spectrapro: document the fact that some grattings might be expressed in µm (instead of nm)

no behaviour change, as it's only comments for potential future improvements
semcomedi: potential improvement
gui: Added histogram and spectogram sliders
Merge branch 'master' of delmic-server.local:odemis
Merge branch 'master' of vpn.delmic.com:odemis
cli convert: cast type to a bigger one if there is potential underflow

16 -17 = 65534 in uint16!
Might sound a rare case, but as most of the images are uint16, this actually happens
with almost any subtraction. This is now casted to int32.