ui/cocoa: Add utility method to check if point is within window

Add a utility method to check whether a point is within the current window
bounds, and use it in the various places in the mouse handling code that
were opencoding the check.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1403516125-14568-3-git-send-email-peter.maydell@linaro.org
