If there is no added printer, disallow closing the window

The state of having no printer added is very weird to the user. Let's prevent that from happening by accident.

Contributes to issue CURA-4736.

resources/qml/AddMachineDialog.qml

@@ -1,4 +1,4 @@
-// Copyright (c) 2016 Ultimaker B.V.
+// Copyright (c) 2017 Ultimaker B.V.
 // Cura is released under the terms of the LGPLv3 or higher.
 
 import QtQuick 2.2
@@ -25,6 +25,15 @@ UM.Dialog
     width: minimumWidth
     height: minimumHeight
 
+    flags: {
+        var window_flags = Qt.Dialog | Qt.CustomizeWindowHint | Qt.WindowTitleHint;
+        if (Cura.MachineManager.activeDefinitionId !== "") //Disallow closing the window if we have no active printer yet. You MUST add a printer.
+        {
+            window_flags |= Qt.WindowCloseButtonHint;
+        }
+        return window_flags;
+    }
+
     onVisibilityChanged:
     {
         // Reset selection and machine name