Double-click for expanding collection root item

[Nikolaj Hald Nielsen]

> Yep, totally agree. I originally had implemented single and double
> click expand toggle. I guess this is a regression caused by Nikolaj's
> refactor.

I thought I made it work like that again. Oh well perhaps I didn't,
but it should be super easy to do no that the code for handling it
only affects the collection browser.

I did notice one bad side effect of making it open/close on single
clicks; when right clicking on the collection header to access the
context menu, it triggers the open close command as well, and starting
a drag does the same. This feels really _wrong_.

- Nikolaj