SLDParser: CssParameter parser should support mixed syntax

SLDParser doesn't currently support CssParameter tags (and other ParameterValueType tags, such as Label) with a mixed content (free text interleaved by element expressions). Using mixed syntax would be very useful, expecially for Label tags, where you could specify something like:

<Label>this is a prefix, <ogc:PropertyName>myProperty</ogc:PropertyName>; this is a postfix</Label>

Included is a patch to org.geotools.styling.SLDParser implementing mixed parsing, modifying the parseCssParameter method.
I have also defined a new version of the method with an additional parameter, trimWhiteSpace (defaulting to true) which permits to parse without trimming whitespace. trimWhiteSpace=false is used for the Label tag parsing (often you need spaces between text and expressions and is not correct to trim them). In all other cases, text is trimmed and completely ignored if empty after trimming.