Message attached property

Create Message attached property to propagate parameters like room, timeline, index and maxContentWidth down to the message content avoiding lots of boilerplate

src/timeline/FetchButtonComponent.qml

@@ -17,18 +17,13 @@ import org.kde.neochat.chatbar
 Delegates.RoundedItemDelegate {
     id: root
 
-    /**
-     * @brief The maximum width that the bubble's content can be.
-     */
-    property real maxContentWidth: -1
-
     /**
      * @brief Request more events in the thread be loaded.
      */
     signal fetchMoreEvents()
 
     Layout.fillWidth: true
-    Layout.maximumWidth: root.maxContentWidth
+    Layout.maximumWidth: Message.maxContentWidth
 
     leftInset: 0
     rightInset: 0