N°2847 - Rework on iUIContentBlock and PageContent

* Add new Trait tUIContentAreas to handle multiple content areas across UIBlock classes (extracted from PageContent)
* Remove return type hinting in some method when type was "self" or the class itself when it's meant to be overloaded (works only with PHP 7.4+)
* Update some PHPDoc
* Reformat some brackets

sources/application/UI/iUIBlock.php

@@ -115,7 +115,7 @@ interface iUIBlock
 	 *
 	 * @return $this
 	 */
-	public function AddHtml(string $sHTML): iUIBlock;
+	public function AddHtml(string $sHTML);
 
 	/**
 	 * Return block specific parameters