Disable "Qt Quick->Preview" action if no qmlviewer in the PATH

An error dialog would be more explicit, but too late because of string
freeze.

src/plugins/qmljseditor/qmljseditorplugin.cpp

@@ -133,7 +133,9 @@ bool QmlJSEditorPlugin::initialize(const QStringList & /*arguments*/, QString *e
     Core::Command *cmd = addToolAction(m_actionPreview,  am, toolsMenuContext,
                    QLatin1String("QtQuick.Preview"), menuQtQuick, tr("Ctrl+Alt+R"));
     connect(cmd->action(), SIGNAL(triggered()), SLOT(openPreview()));
+
     m_previewRunner = new QmlJSPreviewRunner(this);
+    m_actionPreview->setEnabled(m_previewRunner->isReady());
 
     QAction *followSymbolUnderCursorAction = new QAction(tr("Follow Symbol Under Cursor"), this);
     cmd = am->registerAction(followSymbolUnderCursorAction, Constants::FOLLOW_SYMBOL_UNDER_CURSOR, context);

src/plugins/qmljseditor/qmljspreviewrunner.cpp

@@ -24,6 +24,10 @@ QmlJSPreviewRunner::QmlJSPreviewRunner(QObject *parent) :
     m_applicationLauncher.setEnvironment(environment.toStringList());
 }
 
+bool QmlJSPreviewRunner::isReady() const
+{
+    return !m_qmlViewerDefaultPath.isEmpty();
+}
 
 void QmlJSPreviewRunner::run(const QString &filename)
 {

src/plugins/qmljseditor/qmljspreviewrunner.h

@@ -13,6 +13,8 @@ class QmlJSPreviewRunner : public QObject
     Q_OBJECT
 public:
     explicit QmlJSPreviewRunner(QObject *parent = 0);
+
+    bool isReady() const;
     void run(const QString &filename);
 
 signals: