Debugger: Expose mainWindow to other plugins

Needed by GammaRay integration when merging the modes.

Change-Id: I21ee65c55e242238ab0be244c4bb40f1c9c7924e
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>

src/plugins/debugger/debuggerplugin.cpp

@@ -3602,6 +3602,11 @@ void runAction(Id actionId)
     desc.startTool();
 }
 
+QWidget *mainWindow()
+{
+    return dd->m_mainWindow;
+}
+
 void enableMainWindow(bool on)
 {
     dd->m_mainWindow->setEnabled(on);