Select none and all

Add actions for deselecting and selecting all matches.

I thought deselecting everything would be useful for everyone(?), why I also added the default key binding `C-g` to do it. Albeit useful for me, I didn't know if everyone would find the select all action useful and hence didn't add any key bindings for it.

I'm also thinking about whether it'd be even more useful to have a "select all and finish" action.