Switch "Open all in tabs" shortcut to "Ctrl + Alt + a"

I have had a similar problem on another project I worked at ~1 year ago.

I looked at various browsers and found "Ctrl + Alt + " shortcuts to be safest for use by a web app.

I don't remember the full list of browsers that I checked, so this may still need some testing.

Signed-off-by Emilis Dambauskas emilis@emilis.net.

Closes #708 (closed)