modify behavior of position relative tool

This changes the behavior of the "Reset" button in the position relative tool. Instead of setting the x and y coordinates to zero, which isn't very useful, it sets them to the current offset from the target position. This allows you to easily move an object in only one axis if you reset the other axis. The target selection is also modified to allow selecting individual parts of a footprint. Previously, selecting a e.g. pad would select the entire footprint.