Grab key scrolling

[Eben Eliason]

> 1 Scroll events can only be horizontal or vertical, you can't move
> diagonally for example.

This seems like the biggest issue, as diagonal scrolling is one of the
main benefits of the grab approach.  Still, I really want something
mapped to that key.  To clarify, will this even properly generate
horizontal or vertical events?  Does it really know that left to right
is different than top to bottom?

> 2 The precision is limited since you can only move of a certain step.

This is true, but it's not as big an issue to me.  I've worked with
scroll events before, and the event usually just contains a delta
value.  This delta can be handled in any way the handler wants, and so
can easily be scaled up or down, or capped at some max and min values.

> The fact that they are already supported by applications it's clearly a
> big plus though. Maybe for Gen1 we could implement something using
> scrolling events and for later releases aim to add better semantics to
> X. Eben what do you think?

So, I guess the answer is that if this is the best we can do for now,
then we should try it out.  Lack of diagonal scrolling is a big reason
to look for a better way, though.

- Eben