function observeRelevantMutations(oTarget) {
		this._oMutationObserver.observe(oTarget, {
			childList: true,
			subtree: true,
			attributes: true,
			attributeFilter: ["style", "class", "width", "height", "border"],
			characterData: true // also observe text node changes, see https://dom.spec.whatwg.org/#characterdata
		});
	}
Deregister mutation handler for an overlay.
@param {string} sId - Overlay id as registration key