Add collapse-all button on the toolbar

[anilgulecha]

Can a collapse-all button be added next to the sort and refresh icons. I like to collapse all comments and only open up the ones I'm interested in. Save a lot of scrolling effort, and improves user experience.

[anilgulecha]

Just to be clear, this will collapse all non-top level comments. Checkout BaconReader's view and collapse semantics -- when collapsed, the top comment should still be readable -- this is not currently the case in a collapsed comment.

[QuantumBadger]

Just closed #380 as a dupe of this. There should also be an "expand all" menu item to undo the effect, and a preference to hide this from the menu (maybe by default).