Make it so that the menus for individual thread messages can be accessed

This is mostly just for text type messages at the moment but give the framework so that when other message types can be sent in threads they can be added easily

src/timeline/ThreadBodyComponent.qml

@@ -31,11 +31,6 @@ ColumnLayout {
      */
     signal hoveredLinkChanged(string hoveredLink)
 
-    /**
-     * @brief Request a context menu be show for the message.
-     */
-    signal showMessageMenu
-
     Layout.fillWidth: true
     Layout.fillHeight: true
     Layout.maximumWidth: Message.maxContentWidth
@@ -52,7 +47,6 @@ ColumnLayout {
             onHoveredLinkChanged: hoveredLink => {
                 root.hoveredLinkChanged(hoveredLink);
             }
-            onShowMessageMenu: root.showMessageMenu()
             onRemoveLinkPreview: index => threadRepeater.model.closeLinkPreview(index)
             onFetchMoreEvents: threadRepeater.model.fetchMoreEvents(5)
         }