Harmonize link definitions, attributes, and functions in widget-*.xsd's, etc

Currently in widget-screen.xsd there is the definition 'link', with it's associated attributes (and of course associated functions in MacroScreenRenderer.java and htmlScreenMacroLibrary.ftl)

However, in widget-form.xsd this definition doesn't exist (and also not in the .java and .ftl files).
But for linking the definitions hyperlink and sub-hyperlink exists. Which don't exist in either widget-screen.xsd and widget-menu.xsd

And in widget-menu.xsd the definition of link exists, but with less attributes defined than the definition in widget-screen.xsd.

It would be a great improvement if there would be only one definition that is applied everywhere and has the combined functionalities of all defintions currently existing.