Compartment visualization

[zakandrewking]

For new users, the compartmentalization is not at all intuitive. Adding compartment boundaries would be a big help, but it isn't clear to me yet how this should work. We could:

1. Let users manually draw rounded rectangles and circles as compartments
2. Try to automatically find compartment boundaries
3. Draw compartment boundaries on a reaction by reaction basis by putting a dashed line in the background through the middle of the reaction. Then, in theory, Escher could keep track of the closest such lines and connect them when possible.