Implement auxclick events

[pshaughn]

I encountered this because various tests under WPT html/webappapis/scripting want to try every single global event handler to see if on* is connected to addEventListener(*) in the appropriate way; this is one of four missing ones.

Auxclick is just like click, but for buttons that aren't the primary one. https://w3c.github.io/uievents/#event-type-auxclick

[pshaughn]

This needs port-specific code changes to make sure UI clicks are actually sent for non-primary buttons. ports/glutin currently has a special case for just

servo/ports/glutin/headed_window.rs

Line 347 in 85ad53c

MouseWindowEvent::Click(MouseButton::Left, coords.to_f32())

and doesn't send MouseWindowEvent::Click for other buttons.

[chotchki]

So looking at the code here:

servo/ports/glutin/headed_window.rs

Line 332 in 85ad53c

MouseWindowEvent::MouseDown(MouseButton::Left, coords.to_f32())

It looks like it gets either left or right buttons and just emits left only. Would more buttons matter like a middle mouse button?

[jdm]

Yes, we would want to capture any mouse button input.