Additional "SHIFT" drawing constraints (#74)

Some additional ideas for better usability in the diagram editor:

*Hold "SHIFT" when resizing an existing shape (comment, entity, etc.) horizontally or vertically: symmetrically resizes around the center point. (Useful e.g. when I've got relationships laid out straight up/down but need to change the sizes of the targets - right now I have to reposition the target shapes afterwards.)

*Hold "SHIFT" when resizing an existing shape diagonally: maintain initial width-height aspect ratio during resize.

*Hold "SHIFT" when moving an existing shape: constrain movement to horizontal or vertical only (whichever is closest as the mouse moves).