Provide additional modifiers as part of the mouse wheel event

[teddemunnik]

As part of the prototype I'm working on I wanted to be able to access the current mouse position as part of mouse wheel events, in order to implement zoom to mouse cursor. Since we already supply shared modifiers between other mouse events, and from following the discussion in #843, it seemed somewhat reasonable to do this by changing WinHandler::wheel to take a MouseEvent like the other mouse events, and adding wheel_delta to MouseEvent.

I've implemented the changes I am suggesting above as a draft on the windows and GTK backends, but work on the other backends is remaining.

[xStrom]

Generally looks good. A few inline comments.

[teddemunnik]

Thanks for the review @xStrom! I implemented all the changes you requested in the windows shell, let me know if I understood correctly the changes with LOWORD extracting.

[xStrom]

Yeah the LOWORD stuff is mostly fine, but I think let's do it only once.

let system_delta = HIWORD(wparam as u32) as i16 as f64;
let down_state = LOWORD(wparam as u32) as usize;

Then the subsequent use can just be on down_state.

[cmyr]

I think this is reasonable. The only alternative I can think of is to have some general mechanism that exposes the mouse position, and plumb that through EventCtx.

[teddemunnik]

@cmyr getting the mouse position through EventCtx, sounds pretty neat too, and I could imagine that being useful for getting the mouse position at other points in the application, even if this PR would be merged. Looking at Web API WheelEvent inherits from MouseEvent, but adding the wheel event info to the MouseEvent seemed like a simpler approach for rust.

[xStrom]

Looks good, thanks!