Allow creating goals uniformly distributed in a bounding box

This PR makes the "add goals" button in the benchmark GUI show two options: one for creating single goals at the current end-effector pose (the previous default behavior), and another one for creating a set of goals uniformly distributed in a bounding box given by origin and size.

It also switches the goal duplication shortcut from Ctrl-C (which is normally used for copy) to Ctrl-D.