ui/cocoa.m: Fix macOS 10.12 deprecation warnings

macOS 10.12 deprecated/replaced many AppKit constants to make naming
more consistent. Use the new constants, and #define them to the
old constants when compiling against a pre-10.12 SDK.

Signed-off-by: Brendan Shanks <brendan@bslabs.net>
Message-id: 20170425062952.99149-1-brendan@bslabs.net
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>