BUG: Fix spacing buttons in slice controller widget

Ensures automatic spacing button in slice view controller widget toggles automatic spacing on and off, and updates the icon in the widget to reflect the spacing mode. Ensures automatic spacing, manual spacing, and field of view buttons are synchronized with MRML slice nodes. Ensures automatic spacing, manual spacing, and field of view buttons for each of R, Y, G update together appropriately when linked. #2993, in part similar to #2676-2685.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21893 3bd1e089-480b-0410-8dfb-8563597acbee