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/LinkPreviewComponent.qml

@@ -43,18 +43,13 @@ QQC2.Control {
 
     property bool truncated: linkPreviewDescription.truncated || !linkPreviewDescription.visible
 
-    /**
-     * @brief The maximum width that the bubble's content can be.
-     */
-    property real maxContentWidth: -1
-
     /**
      * @brief Request for this delegate to be removed.
      */
     signal remove(int index)
 
     Layout.fillWidth: true
-    Layout.maximumWidth: root.maxContentWidth
+    Layout.maximumWidth: Message.maxContentWidth
     Layout.minimumHeight: root.defaultHeight
 
     leftPadding: 0