Add a placeholder for a pattern in the header, if the theme wants to add one.

resources/qml/Cura.qml

@@ -123,6 +123,17 @@ UM.MainWindow
                     }
                 }
             }
+
+              // This is a placehoder for adding a pattern in the header
+             Image
+             {
+                 id: backgroundPattern
+                 anchors.fill: parent
+                 fillMode: Image.Tile
+                 source: UM.Theme.getImage("header_pattern")
+                 horizontalAlignment: Image.AlignLeft
+                 verticalAlignment: Image.AlignTop
+             }
         }
 
         MainWindowHeader