Enforce `Event => Unit` Event handlers

[oyvindberg]

Wouldn't it be better if we enforced that event handlers return Unit?
That is change this:

var onmousedown: js.Function1[MouseEvent, _] = js.native

to this:

var onmousedown: js.Function1[MouseEvent, Unit] = js.native

If you agree i can for sure throw up a PR with the necessary changes

[lihaoyi]

I don't remember why I did this initially, but IIRC there was some odd interaction which made type-inference sad if i made it have an explicit Unit. @sjrd do you remember?

[sjrd]

It would be worse. The Any/_ result type is better. It allows to use a lambda that returns anything, not only lambdas that return Unit. This is similar to how foreach in the Scala collections accepts functions that return anything.