Add support for indicating than an XML element is deprecated

[odrotbohm]

To safely deprecate XML elements from namespaces it would be nice to be able to express that deprecation in the XSD for the namespace. Spring already defines a tool namespace that contains information to let IDEs evaluate information to apply additional verifications, show warnings etc.

A new <tool:deprecation …> element could take a message attribute that'd allow XSD authors to mark an element as deprecated and provide additional information to give users a hint to what to use instead and also when they have to expect the eventual removal of that element.

@martinlippert indicated he'd already be looking into what it'd take to let the current generation of STS use such element. I guess it's worth pinging the IntelliJ folks as well.

[jhoeller]

Since we intend to phase out XML bean definitions and in particular XML configuration namespaces, it does not seem necessary to introduce a deprecation mechanism for individual elements anymore.