use [ITAddressBookMgr encodeColor]

Use [ITAddressBookMgr encodeColor] in _exportColorPresetToFile
use decodeColor in _loadPresetColors
allow the themes to specify if they want sRGB
Make sure our fallback is an RGB color

[NSColor blackColor] returns a color in NSGrayColorSpaceModel, which has
no red, blue or green components, and will raise an exception if they
are requested. Instead return black in NSRGBColorSpaceModel.
Always return colors in NSCalibratedRGBColorSpace
Minor tweaks