[FEATURE] Fixed painting and color picking on global selections

This patch fixes three problems:

1) Painting on a global selection now is not recursively limited to
the selected area of the selection itself. If current node and active
selection mask are the same entity, KisResourcesSnapshot will report
no selection active.

2) Color Picker can now pick from selection masks. Yes, it converts the
color into compositionSourceColorSpace() to make it paintable.

3) The Color Picker's option widget now shows alpha value correctly,
although the resource server gets opaque color.