Migration of viewport legend to legend component module
Inclusion of the legend module mentioned in the previous commit
gui: fix failure on some specific scaling values

sometimes the floating errors and rounding accumulate, and it goes outside
of the asserting by 1 px. I've seen math.ceil(122.0)-int(0)=123.0. So allow
to recover correctly from these errors by shortening back by 1 px.
gui slider: allow even narrower selection for range slider

it's still kind of usable with 2 px, and it's quite useful when analysing
spectra.
Merge branch 'master' of vpn.delmic.com:odemis