Hold "shift" to constrain drawing modes (#66)

Can you provide a feature where, when drawing custom shapes or arrows, holding SHIFT constrains the drawing? For lines, holding SHIFT would snap the line to horizontal, 45 degree, or vertical slope (whichever is closest to the mouse cursor). For ellipses, holding SHIFT would force a circle; likewise, for rectangles, SHIFT would force a square. And for relationship arrows, holding SHIFT would have the same effect as for a custom line (horizontal, 45 degree, or vertical slope). For example, look at shape drawing in most graphics programs, MS Word/Visio, etc.