Changed the effect of the contract button, added interactive delete and contract modes
Changed the effect of the contract button, added interactive delete and contract modes

The contraction button now contracts edges (independently) if only
edges are selected, identifies all selected vertices if only vertices
are selected, toggles interactive edge-contraction mode if nothing is
selected, and does nothing otherwise.  Added a more robust system for
switching between modes.  Fixed bug where the edge addition wouldn’t
check if the edge was new before adding it.

JZacharyG/StickyDraw,JZacharyG/StickyGraph,JZacharyG/StickyGraph