Add command for setting multiple icon overlays and workaround AppleEvent timeout issue with 10.8.2

Fixes an AppleEvent timeout issue with OS X 10.8.2 described here: http://brian-webster.tumblr.com/post/32830692042/a-workaround-for-aesendmessage-hanging-on-os-x-10-8-2

Also added methods to set multiple icons. This drastically improves performance when updating many icons.