Extract interface for weight for CategoryItem and TemplateItem

--> allows to extract a common Comparator from org.fusesource.ide.projecttemplates.wizards.pages.model.CategoryItem.addTemplate(TemplateItem) and org.fusesource.ide.projecttemplates.wizards.pages.model.CategoryItem.addSubCategory(CategoryItem) and org.fusesource.ide.projecttemplates.wizards.pages.model.TemplateModel.determineCategoryExtension(IConfigurationElement)
--> with comparator extract, allows to write unit tests on this comparator