toggle option to prevent disconnected surfaces

[lsyeazell]

new feature as suggested by the coauthor problem

[edemaine]

Thanks for working on this! I gave it a try, and have the following comments:

1. I find the user interface unclear. The button seems to be labeled with what you'd transition to, as opposed to the current mode. Maybe a checkbox or toggle of some sort would be clearer?
2. Actually, the software doesn't seem very consistent when allowing disconnection. Sometimes I get a disconnected piece, but sometimes it just ignores the fact that I've created a cycle of cuts (some of the cuts are ignored). So perhaps the new mode should be permanently on (and doesn't need a UI), at least until this is fixed?
3. If the surface is already disconnected (from the other mode), then in the no-disconnect mode, I can't click anything. Ideally we'd just check whether the two faces being cut are otherwise connected, and allow it if so.