This class provides a simple javax.swing.filechooser.FileFilter
implementation that accepts files based on their extension, or
if they are a directory (to allow traversal of the filesystem).
Sets the desired velocity of this object - the SimWorld will then set its
actual velocity accordingly (depending on wheher the object is colliding
with another object etc.) Positive values mean the objet wants to move
forward, and negative values mean the object wants to move backward.