[style dock] fix defining min/max values via raster histogram

[nirvn]

Description

This RP resurrects the setting of raster band min/max values through the style dock's histogram tab. It also switch the computation min / max widget method to "user defined" (which prevents values from being reverted to default when changing saturation / contrast / etc. values).

@rouault , @NathanW2 , are you OK with the proposed change?

P.S. @rouault , as you might have noticed, I filed a bug yesterday on min / max values broken when opening a raster layer's properties window (http://hub.qgis.org/issues/16353). This PR doesn't fix that bug, it's still in need of your attention 😄

Checklist

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[NathanW2]

I'm happy if you are happy. I don't do raster stuff so happy to take your lead on the workflow here.

[nirvn]

@NathanW2 , I'm happy, therefore we are happy.

[rouault]

@nirvn ef26d95 might fix some of the problems you've seen