Add grey border between extruder boxes

This is done by fitting tightly a rectangle around the grid of boxes. The boxes themselves have a white background but there is spacing between the boxes, which results in the little border.

Contributes to issue CURA-3161.

resources/qml/PrintMonitor.qml

@@ -68,6 +68,12 @@ Column
         }
     }
 
+    Rectangle
+    {
+        color: UM.Theme.getColor("sidebar_lining")
+        width: parent.width
+        height: childrenRect.height
+
         GridLayout
         {
             id: extrudersGrid
@@ -107,6 +113,7 @@ Column
                 }
             }
         }
+    }
 
     Loader
     {