Expand/collapse sets with single click instead of double

[jacksleight]

I think it would be nice if you could expand/collapse a set with a single click on the label instead of a double click, along with a pointer cursor on hover to indicate that you can do something.

When you have a lot of nested sets and fields it really helps to keep things tidy if they're all collapsed by default, but you also want to be able to get into those sets quickly. Currently double clicking the label is the fastest way, but single clicking would be faster!

Single clicking on the label doesn't currently do anything, so I don't think this would cause any problems.