Add MouseCursor::pos and fix move_rel and mov_abs

[benediktwerner]

As discussed in #16.

Not sure if the name is good. Maybe get_pos would be better?

Also, the MouseCursor functions taking self is a bit awkward. Maybe it would be better to just make them associated methods? Or at least make it Copy?

Edit: Also, I'm using MaybeUninit since I saw it used somewhere else in the code but I just realized that the other place is using MaybeUninit::zeroed. I think it should be safe as is and maybe even slightly better for performance (although the difference is probably minimal) but if you want I can change it. Also, maybe it would be clearer (and work in earlier Rust versions?) to use std::mem::zeroed()?

Edit2: Closes #16

[obv-mikhail]

Just pos seems fine to me.

Yeah, the self should probably be removed for the MouseCursor functions..

I am ok with any option for for the point.

[benediktwerner]

Ok, I removed the self from the MouseCursor and MouseWheel methods for both the windows and Linux implementations and also updated the example and README.

I think this should be everything now. Keep in mind that this PR is full of breaking changes so the next release should increase the version appropriately, probably to 0.5.0.