Allow arbitrary attributes on DOM elements in generated XSD

[slachiewicz]

Reworked version of #483, which fixes #482.

A DOM field maps to an element carrying free-form XML, so DOM merge hints may appear on it:

<configuration combine.self="override">
  <goalPrefix combine.self="override"/>
</configuration>

The generated xs:complexType declared no attributes, so this failed schema validation even though Maven accepts it. Nested elements were already covered by the existing xs:any processContents="skip"; only the element itself was missing an attribute wildcard.

Changes

• XsdGenerator emits <xs:anyAttribute processContents="skip"/> for DOM fields.
• writePropertiesElement is shared by DOM and java.util.Properties fields, so it now takes a flag. A Properties container element never carries attributes, so it keeps the stricter schema and typos there are still reported.
• New DomXsdGeneratorTest, following FeaturesXsdGeneratorTest: it generates the schema, validates a document using combine.self on the DOM element, and asserts that the same attribute on a Properties element is still rejected. The test fails without the generator change.

Differences from #483

• Rebased onto current master — that PR predates the test migration in chore: migrate junit 3 test to junit4 #523 and no longer compiles (lookup(ModelloCore.ROLE), no @Test/@PlexusTest).
• The wildcard is restricted to DOM; add <xs:anyAttribute processContents="skip"/> to DOM type's complexType generated (in xsd generation) #483 also loosened Properties.
• Assertions added; add <xs:anyAttribute processContents="skip"/> to DOM type's complexType generated (in xsd generation) #483 only checked that generation did not throw.
• The 3328-line copy of Apache Maven's maven.mdo is replaced by a 42-line dom.mdo, and the test gets its own output directory instead of sharing target/generator-results/xsd with XsdGeneratorTest.

Credit to @XenoAmess for the diagnosis and the original patch.