ui/cocoa.m: Make a better about dialog

The about dialog in QEMU on Mac OS X is very plain and unhelpful. This patch
makes the about dialog look a lot better and have some descriptive information
on what version of QEMU the user is running.

Signed-off-by: John Arbuckle <programmingkidx@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: ED59936E-3EB2-46AB-9E33-AB26E382B884@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>