This commit implements the improvements described at:
https://lists.osgeo.org/pipermail/qgis-developer/2016-September/044393.html
The QgsRasterMinMaxWidget now offers a seetting to specify that the statistics
should be computed each time the canvas extent changes.
Other changes:
- the content of the QgsRasterMinMaxWidget is now persistant.
- there is no longer any Load button. The global Apply / OK button of the raster
properties dialog has this effect.
- the default "limits" for single band raster is now MinMax and not CumulativeCut
- the default "limits" can be configured for single band, multi band single byte and
multi band multi byte
- "Strech using current extent" honours the "limits" instead of forcing min/max.
