select_all and unselect_all for atk4_checkboxes

Two new methods for ui.atk4_checkboxes. These can be handy for some developers. It's a small part of one of my upcoming add-ons.

But if you think it's not useful in core, then I'm fine. I'm using my own js file in add-on anyway :)
Merge pull request #203 from DarkSide666/patch-15

select_all and unselect_all for atk4_checkboxes