feat(page-element): unique id for each page-element

The id is based on the location of the element in the element tree

src/store/page/page-element.ts

@@ -189,6 +189,23 @@ export class PageElement {
 		return this.parent.children.indexOf(this);
 	}
 
+	/**
+	 * Returns the unique ID of this element
+	 * @return an array of numbers which represents the unique ID
+	 */
+	public getId(): number[] {
+		if (!this.parent) {
+			return [0];
+		}
+
+		if (!this.parent.parent) {
+			return [0, this.getIndex()];
+		}
+
+		const parentID = this.parent.getId();
+		return [...parentID, this.getIndex()];
+	}
+
 	/**
 	 * Returns the parent page element if this is not the root element.
 	 * @return The parent page element or undefined.