ui/cocoa.m: fix help menus

Make the help menus actually work. The code will search thru three different
locations for the help file. If it can't be found a dialog will tell the user
the file can't be found.

Signed-off-by: John Arbuckle <programmingkidx@gmail.com>
Message-id: F6B689F9-4DBD-4C50-BC38-35E5DD03D396@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>