Remove geomViewportNudge ?

[bhousel]

Continuing with #263 the goal of paring this down to just the essentials that might be used to build other tools..

geomViewportNudge is kind of an implementation detail - it takes a mouse point and returns a vector to pan the viewport. It's only used in one behavior in Rapid and the code contains some magic numbers that only apply to Rapid's toolbars.