Support zooming windows

[phillco]

The "zoom" behavior (equivalent to double clicking on the titlebar) is still programmatically invokeable as an extra action on the fullscreen button [1]:

window.element.children.find_one(AXSubrole='AXFullScreenButton').perform("AXZoomWindow")

This would be handy to have, since in many applications it resizes the window to the content size, which is otherwise not easily doable with the snap commands.

[1] Props to this Hammerspoon issue for the source: Hammerspoon/hammerspoon#181