Extended WMS Layer Metatadata 

Improves support for WMS 1.1.1/1.3.0 Layer metadata handling:

* Lookup dataset metadata from services metadata configuration
* Add requestable flag to Identifier element in themes configuration, needed for unrequestable group layers
* Merge metadata from layer and themes configuration
* Much extended list of metadata options (see below)
* Refactored capability writers for WMS 1.1.1./1.3.0 layers, including tests

This solves #445.

```xml
<deegreeServicesMetadata configVersion="3.2.0" xmlns="http://www.deegree.org/services/metadata" 
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
  xsi:schemaLocation="http://www.deegree.org/services/metadata http://schemas.deegree.org/services/metadata/3.2.0/metadata.xsd">

  <DatasetMetadata>
    <MetadataUrlTemplate>http://www.nationaalgeoregister.nl/geonetwork/srv/nl/csw?service=CSW&amp;request=GetRecordById&amp;version=2.0.2&amp;id=${metadataSetId}</MetadataUrlTemplate>
    <ExternalMetadataAuthority name="a">http://www.aaa.net</ExternalMetadataAuthority>
    <ExternalMetadataAuthority name="b">http://www.bbb.net</ExternalMetadataAuthority>
    <Dataset>
      <Name>layer1</Name>
      <Title>Layer 1 Title (from metadata config)</Title>
      <Abstract>Layer 1 Abstract (from metadata config)</Abstract>
    </Dataset>
    <Dataset>
      <Name>layer2</Name>
      <Title>Layer 2 Title (from metadata config)</Title>
      <Abstract>Layer 2 Abstract (from metadata config)</Abstract>
    </Dataset>

    <Dataset>
      <Name>grouplayer</Name>
      <Title>Group Layer Title (from metadata config)</Title>
      <Abstract>Group Layer Abstract (from metadata config)</Abstract>
      <Keywords>
        <Keyword>keyword1.1</Keyword>
        <Keyword>keyword1.2</Keyword>
        <Type codeSpace="codeSpace1">type1</Type>
      </Keywords>
      <Keywords>
        <Keyword>keyword2.1</Keyword>
        <Keyword>keyword2.2</Keyword>
        <Type codeSpace="codeSpace2">type2</Type>
      </Keywords>      
      <MetadataSetId>fff-fff-fff</MetadataSetId>
      <MetadataURL>http://metadataurl1.net</MetadataURL>
      <MetadataURL>http://metadataurl2.net</MetadataURL>
      <ExternalMetadataSetId authority="a">aaa-aaa-aaa</ExternalMetadataSetId>
      <ExternalMetadataSetId authority="b">bbb-bbb-bbb</ExternalMetadataSetId>
      <DataURL format="text/xml">http://www.data.de/xml</DataURL>
      <DataURL format="text/plain">http://www.data.de/plain</DataURL>
      <FeatureListURL format="text/xml">http://www.featurelist.de/xml</FeatureListURL>
      <FeatureListURL format="text/plain">http://www.featurelist.de/plain</FeatureListURL>
      <Attribution>
        <Title>Attribution title</Title>
        <URL>http://attribution.de</URL>
        <LogoURL format="image/png" width="200" height="200">http://logo.de</LogoURL>
      </Attribution>
    </Dataset>

  </DatasetMetadata>
</deegreeServicesMetadata>
```