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/bitcoingui.h

@@ -115,7 +115,7 @@ public slots:
     void setEncryptionStatus(int status);
 
     /** Notify the user of an error in the network or transaction handling code. */
-    void error(const QString &title, const QString &message);
+    void error(const QString &title, const QString &message, bool modal);
     /** Asks the user whether to pay the transaction fee or to cancel the transaction.
        It is currently not possible to pass a return value to another thread through
        BlockingQueuedConnection, so an indirected pointer is used.