[ux] Auto dismiss message bar items when a link is clicked in the item

At this stage the user has already read and acted upon the notification,
so requiring them to explicity dismiss the notification is an
annoying extra action.

Author: nyalldawson

src/gui/qgsmessagebaritem.cpp

@@ -284,4 +284,5 @@ void QgsMessageBarItem::urlClicked( const QUrl &url )
     QgsGui::instance()->nativePlatformInterface()->openFileExplorerAndSelectFile( url.toLocalFile() );
   else
     QDesktopServices::openUrl( url );
+  dismiss();
 }