qtui.h/noui.h interface cleanup

- rename wxMessageBox, remove redundant arguments to noui/qtui calls
- also, add flag to force blocking, modal dialog box for disk space warning etc
- clarify function naming
- no more special MessageBox needed from AppInit2, as window object is created before calling AppInit2

src/qt/clientmodel.h

@@ -52,7 +52,7 @@ signals:
     void numBlocksChanged(int count);
 
     //! Asynchronous error notification
-    void error(const QString &title, const QString &message);
+    void error(const QString &title, const QString &message, bool modal);
 
 public slots: