ui/cocoa.m: add Speed menu

Programs running inside of QEMU can sometimes use more CPU time than is really
needed. To solve this problem, we just need to throttle the virtual CPU. This
feature will stop laptops from burning up.

This patch adds a menu called Speed that has menu items from 100% to 1% that
represent the speed options. 100% is full speed and 1% is slowest.

Signed-off-by: John Arbuckle <programmingkidx@gmail.com>
Message-id: D6FAAABF-064D-49C0-B572-C73679F34052@gmail.com
[PMM: Moved "mark 100% menu item as checked initially" code to
 after menu item is allocated, not before it]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
