Move Welcome Page to separate plugin.

src/plugins/coreplugin/coreplugin.h

@@ -32,10 +32,13 @@
 
 #include <extensionsystem/iplugin.h>
 
+namespace Core {
+    class IMode;
+}
+
 namespace Core {
 namespace Internal {
 
-class WelcomeMode;
 class EditMode;
 class MainWindow;
 
@@ -56,7 +59,7 @@ public slots:
 
 private:
     MainWindow *m_mainWindow;
-    WelcomeMode *m_welcomeMode;
+    Core::IMode *m_welcomeMode;
     EditMode *m_editMode;
 };