qt: Unify AboutDialog and HelpMessageDialog

They share so much code and functionality that they may as well
be one class.

src/qt/bitcoingui.cpp

@@ -549,8 +549,7 @@ void BitcoinGUI::aboutClicked()
     if(!clientModel)
         return;
 
-    AboutDialog dlg(this);
-    dlg.setModel(clientModel);
+    HelpMessageDialog dlg(this, true);
     dlg.exec();
 }