Change icon of manager-osx

[JoshH99]

The manager-osx.app icon isn't that great to be honest. It doesn't say "this is XAMPP" right away like the Windows tray icon does. So I changed the configuration on my system to use a vectorized XAMPP logo and converted it to an ICNS file that's usable on the system. It also looks a lot better in my dock than the other whatever-it-was.

https://www.dropbox.com/s/sj41fr15ce4ig3x/installbuilder.icns --- the file in my Dropbox account if someone's interested in updating the icon.