IDE-1503 Integrate xml-search into Liferay-IDE

- simplify validtion preferences to five types
- create a new class LiferayDescriptorBaseValidator insteadof DefaultDOMNodeValidator as
  the base validator of all descriptor validators
- add liferay custom markers and validators
IDE-1503 Intergate xml-search into Liferay-IDE

- Moved descriptors and reference declarations from ui to core plugin
- Ingore the unit tests about descriptor validation which can only
  run in Eclipse workbench temporarily
IDE-1503 Intergate xml-search into Liferay-IDE
  -- move the contents of xml-search-core to xml-seach-ui
  -- removed related unit tests