Add the 'opacity' property to CSSStyleDeclaration

(CSSStyleDeclarationImp::getPropertyID): Add 'opacity'
(CSSValueParser::CSSValueParser): Support 'opacity'
(CSSStyleDeclarationImp::opacity, CSSStyleDeclarationImp::getOpacity, CSSStyleDeclarationImp::setOpacity): New
(CSSStyleDeclarationImp::CSSStyleDeclarationImp, CSSStyleDeclarationImp::getProperty, CSSStyleDeclarationImp::specify, CSSStyleDeclarationImp::reset, CSSStyleDeclarationImp::compute): Support 'opacity'
(CSSNumericValue::clip, CSSNumericValueImp::clip): New
(CSSNumericValueImp::getCssText): Change the unit to CSS_NUMBER
(CSSNonNegativeLengthImp): Rename from CSSNonNegativeValueImp to make it clear this property is for length values; all users changed
Makefile.am (idl_files, EXTRA_DIST): Add css3color.idl
idl/css3color.idl: New