some theming issues

I have been having problems with theming, "#prefix" was being printed twice, and the content of $element['#id'] was being set twice. Both these issues have been seen before, see #1274446: #prefix and #suffix doesn't work with jquery_colorpicker and #1903808: Duplicate CSS ids

The attached patch on current dev fixes these issues, not tested with jquery_colorpicker in 'Field' mode but I doubt that it will cause any problems.

IMO these two fixes make this cool tool that much more useable ;-)