Let welcomeDialog default to invisible

This speeds up the bootup a bit since it doesn't have to create a number
of the items in the dialog

resources/qml/Cura.qml

@@ -74,7 +74,7 @@ UM.MainWindow
     WelcomeDialogItem
     {
         id: welcomeDialogItem
-        visible: true  // True, so if somehow no preferences are found/loaded, it's shown anyway.
+        visible: false
         z: greyOutBackground.z + 1
     }