Refine the Doclet APIs

The Doclet related APIs specifically DocletEnvironment.java needs some adjustments based on the doclets use case, and is basically
a fallout from the modularization work see JDK-8159305

1. Simplify getSelectedItems() to getSelectedItems(Kind.{MODULE, PACKAGE, TYPE})
2. Add isVisible(Element)
3. Simplify getIncludedModulesElements(), getIncludedPackageElements(), getIncludedTypeElements(), to getIncludedElements((Kind.{MODULE, PACKAGE, TYPE})

Fixup the calls to these and provided shouldDocument(Element) utility for stddoclet's internal use.