ui/cocoa.m: Prevent activation clicks from going to guest

When QEMU is brought to the foreground, the click event that activates QEMU
should not go to the guest. Accidents happen when they do go to the guest
without giving the user a chance to handle them. In particular, if the
guest input device is not an absolute-position one then the location of
the guest cursor (and thus the click) will likely not be the location of
the host cursor when it is clicked, and could be completely obscured
below another window. Don't send mouse clicks to QEMU unless the
window either has focus or has grabbed mouse events.

Reported-by: John Arbuckle <programmingkidx@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: John Arbuckle <programmingkidx@gmail.com>
Message-id: 1448551168-13196-1-git-send-email-peter.maydell@linaro.org
