ENH: Added segment selector combobox to slice view controller

A fourth row is added to slice view controller which is only visible if a segmentation node is present in the scene. It behaves differently than the other rows in the sense that it does not assign the selected node to a role that is specific to the slice view, but allows changing display properties of a segmentation that are global (visibility, opacity, segment visibility, outline/fill). Selecting a segmentation shows it if hidden.

ENH: Added multi-selection option to segment selector widget

ENH: Removed legacy labelmap support

Segmentation node is no longer subclass of labelmap volume node. Temporary color table is removed (color index tag remained, consecutive values are assigned). Utility function ShiftImageDataExtentToZeroStart moved to volume node from segmentation node, may come in handy