Use CPTextFields for RGB, HSB, and hex inputs in CPSliderColorPicker
The old DOM based fields only accepted a few characters. Backspace and
other expected functionality was broken. Switching to CPTextFields
makes them behave as expected.
Fix for a few minor regressions:
1. Split out flatten specific code to get file size back down.
2. Fix for non-native string URL not working with the URLCache (typeof x !=== "string")
3. Fix for slight IE performance regression.
Reviewed by me.