Allow handling clickable components / three.js object instances as <a> and <button>

[vatro]

Atm the preventDefault modifier makes no sense with "click" related PointerEvents because we're generating only <div> shadow DOM elements which have no default browser action.

• generate shadow DOM <a> elements if a component / three.js object instance is / should be handles as a link. -> link prop (an Object containing <a> properties)
• generate shadow DOM <button> elements if a component / three.js object instance is / should be handled as a button. -> button prop (an Object containing <button> properties)