Format.SLD: add support for TextSymbolizer

It does not really make sense to implement read support right now, since vector layers can't render labels as yet.

Write support makes sense IMHO as you can now easily label a WMS layer using SLD.

An example useage is:

    var symbolizer = {};
    symbolizer["Text"] = {property: propertyName, fontFamily: fontFamily,
        fontSize: fontSize, fillColor: color};

Which will create something like (when used in an SLD document):

<StyledLayerDescriptor xmlns="http://www.opengis.net/sld" version="1.0.0" xsi:schemaLocation="http://www.opengis.net/sld http://schemas.opengis.net/sld/1.0.0/StyledLayerDescriptor.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><NamedLayer><Name>AAA64</Name><UserStyle><FeatureTypeStyle><Rule><Name>default</Name><TextSymbolizer><Label><ogc:PropertyName xmlns:ogc="http://www.opengis.net/ogc">ROUTE</ogc:PropertyName></Label><Font><CssParameter name="font-family">Arial</CssParameter><CssParameter name="font-size">10</CssParameter></Font><Fill><CssParameter name="fill">#FFFF00</CssParameter></Fill></TextSymbolizer></Rule></FeatureTypeStyle></UserStyle></NamedLayer></StyledLayerDescriptor>

patch implementing write support for TextSymbolizer

write text symbolizers with support for label expressions

Hey Bart - This is a nice addition. I think we'll eventually want to support more ogc:expressions. With that in mind, I decided a "label" property in the text symbolizer would more closely fit the sld:Label element. This parser has support for limited expression handling, so you can do something like:

var symbolizer = {
    "Text": {
        "label": "There are ${population} people in ${state}.",
        "fontFamily": "Arial" // etc
    }
};

I also added a test to demonstrate it working. Let me know if this works for you.

Great work Bart and Tim! I can confirm that tests pass in FF2 and IE7, and that the patch does not interfer with existing code. Also, is an important contribution to make SLD read/write support more complete. Tim, please commit.

(In [7333]) Adding support for TextSymbolizer writing in the SLD format. Thanks to Bart for the original patch. This provides basic expression handling for text labels. Read support later. r=ahocevar (closes #1542)