ui/cocoa.m: replace scrollingDeltaY with deltaY

The NSEvent class method scrollingDeltaY is available
for Mac OS 10.7 and newer. Since QEMU supports Mac OS
10.5 and up, we need to be using a method that is
available on these version of Mac OS X. The deltaY
method is a method that does almost the same thing as
scrollingDeltaY and is available on Mac OS 10.5 and
up. So we can replace scrollingDeltaY with deltaY.

We only check deltaY's value if it is not zero
because zero means that the scrolling increment was
sufficiently fine that it was only reported in scrollingDeltaY,
or that the scrolling was horizontal.

Signed-off-by: John Arbuckle <programmingkidx@gmail.com>
Message-id: 20180709150235.7573-1-programmingkidx@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: tweak commit message and comment a little]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

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