Snapping Preferences -- spinboxes, not sliders

Summary:

Document Properties' Snap preferences should be changed to spinboxes (or at minimum use the same slider/spinbox combination as Preferences' Interface and Snapping preferences):

• Sliders don't allow quick, accurate settings (unless they have demarcations, which they don't)
• They are the sole example of that GUI element in Inkscape. They are not even similar to the Blur and Opacity settings slider/text-entry boxes used in the various dialog panels.