Contacts/Clashes improvements

[Elaine Meng]

Now I'm thinking:

• don't want to make default treatment "select" because then you lose your original selection

Further thoughts lead me down a rabbit hole:

• it is dumb to use Contacts without a selection
• in that case should turn on "Limit by selection" by default, probably with "at least one end selected" which is already its default suboption
• but then if you try to use it without a selection when "limit by selection" is on, there is an annoying error dialog. Should instead be a warning that doesn't raise a dialog and that is more understandable, e.g. "there is no selection; please select atoms to limit the calculation"

I also learn toward "reveal true" default in both Contacts and Clashes and their commands, but similar to changing the selection, it changes the state (of display at least, not quite as bad as selection) and is not easy to undo. At least currently "undo" does not re-hide anything that was shown by contacts/clashes.

Thus perhaps you should ignore this whole rabbit-hole unless you agree with some of it enough to want to make a change.