Graphical tooltips for sub-models and functions

The Module Tool now shows a little diagram at the bottom right whenever the user hovers above a connection that has a hidden model inside. The image is regenerated whenever the inner model changes.

The decorator icon is also modified so that it is consistently the same icon for all connections that have a hidden model inside. This is due to confusions caused by the inconsistent meanings of the other decorators.

In the next screenshot, the tool-tip window displays the function that is used to assign the value to the repressor binding site. This is important because TinkerCell allows different modeling strategies, so the user (quickly) should know when an assignment rule is used.