Added CWSystemInfo

- JS can now detect the PPI of iPad’s and iPhone’s via CWSystemInfo.PPI() - all other devices will return the 96 DPI for now.
- Fixed a bug in the PPI detection of retina displays. Since they are retina, they have double the PPI of non-retina displays, but effectively use the same amount, so the same amount of PPI is now returned for retina and non-retina displays
- The CWDevice was moved to common, which was forgotten in the last commit