Rectangular panel extrusions (and negatives) can now be created about a single centre point in the 3D View
When selected, the user is prompted to select the origin of the rectangle i.e. the Centre point.
The user is then prompted to specify the width of panel (X)
Finally, the user is prompted to specify the height of the panel (Y).
Negatives are placed in a similar manner
The specified negative is removed from the selected panel.
