new viewport following function; moves the viewport so that the last placed
node is in the center. the idea is to greatly improve tracing of long, linear
features from imagery or tracks (no need to right-click and pan all the time),
however the execution still leaves to be desired; problem is that after you
have placed a node you have to wait for the movement to finish before you can
place the next node. possible enhancements: only scroll if placed node is on
the outer rim of viewport?