Add constants for tags and attributes in xpp3

• Complexity:

  Intermediate
• Testcase included:

  yes
• Patch Submitted:

  Yes

I proposed to add constants for tags and attributes, i.e.:

public class MavenXpp3Reader {
...
    /**
     * Tag name for <code>myTag</code>.
     */
    public static final java.lang.String TAG_MYTAG = "myTag";

    /**
     * Attribute name for <code>myAttribute</code>.
     */
    public static final java.lang.String ATTRIBUTE_MYATTRIBUTE = "myAttribute";
...
}

The patch includes:

• added new methods in Xpp3ReaderGenerator class to handle that: addConstantField(...) and getConstantName(...)
• replaces all tagName/singularTagName by getConstantName(...)
• removed unused import
• updated Xpp3WriterGenerator to use the constants defined in the generated Xpp3Reader class
• added a test case

Note: in src/test/java, the package org.codehaus.modello.generator.xml.xpp3 was moved to org.codehaus.modello.plugin.xpp3 like defined in src/main/java