Modals without duplicate event listeners

[skitterm]

This fixes the "duplicate modal listener" issue detailed in #954.

Similar to the pattern in dropdowns, check if the click event is already there before adding it to the modal toggles.

Note that toggleClick is moved outside the function modal. Otherwise, when calling modal() multiple times, duplicate events are getting bound. This is because the toggle click method is a different instance of the function each time modal() is called. Sticking it outside of the modal function ensures that it's the same instance of the function used by all modals (so the e.fn === toggleClick conditional is true when we'd expect).

I wasn't sure what to do with the else case (when node is passed to bindModals). I didn't see any usage of bindModals being called with node as an argument, but maybe that's in there for user customization? If it needs to remain there, it will need a similar "if the event exists, don't add it again" logic.