VariableManager: Add support for javascript varibales

Add JsExpander that can be used to register additional functionality
in its qscriptengine and to evaluate expressions.

Register that with the VariableManager, using the "JS" prefix.

Change-Id: I7d8f1ddc3484104f3943685dbac09e786d60212e
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/coreplugin/mainwindow.h

@@ -55,6 +55,7 @@ class DocumentManager;
 class HelpManager;
 class IDocument;
 class IWizardFactory;
+class JsExpander;
 class MessageManager;
 class MimeDatabase;
 class ModeManager;
@@ -172,6 +173,7 @@ private:
     MessageManager *m_messageManager;
     ProgressManagerPrivate *m_progressManager;
     VariableManager *m_variableManager;
+    JsExpander *m_jsExpander;
     VcsManager *m_vcsManager;
     StatusBarManager *m_statusBarManager;
     ModeManager *m_modeManager;