Added blueComponent to NSColor and use the correct color for NSWindow.transparency.

By suggestion of Ulrich Bogun on the Xojo forums
- Added missing 'blueComponent' to NSColor.
- Changed NSWindow.Transparancy method to use the window's actual background color rather than an aproximation.

(https://forum.xojo.com/13084-correct-window-transparency-macoslib-can-t-use-github/0)