Flat Wizard should allow setting different gain values for different filters

[Shlomi Vaknin]

When doing flats for multiple filters, there should be a way to customize the gain for the different filters. Currently, gain is dominated by the luminance filter requirements, which makes flat exposures really long for narrowband filters.

(as discussed in #401 with @Igor von Nyssen )

[Stefan B]

will be possible in 3.x