Set the flag of the welcome dialog to Dialog

We want the dialog to be modal, but if it's a window (and not a dialog) the
modality can't be set (or well, it can be set, but it's just ignored at that point)

CURA-6057

resources/qml/WelcomePages/WelcomeDialog.qml

@@ -20,7 +20,7 @@ Window
     id: dialog
     title: catalog.i18nc("@title", "Welcome to Ultimaker Cura")
     modality: Qt.ApplicationModal
-    flags: Qt.Window | Qt.FramelessWindowHint
+    flags: Qt.Dialog | Qt.FramelessWindowHint
 
     width: 580 * screenScaleFactor
     height: 600 * screenScaleFactor