QML UI: add 2cm x 2cm square to theme test page

This should make it easier to spot displays that are completely out of
whack in their scaling.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

mobile-widgets/qml/ThemeTest.qml

@@ -109,6 +109,15 @@ Kirigami.Page {
 			text: Number(font.pixelSize / Screen.pixelDensity).toFixed(2) + "mm"
 		}
 
+		Kirigami.Label {
+			text: "2cm x 2cm square:"
+		}
+		Rectangle {
+			width: Math.round(Screen.pixelDensity * 20)
+			height: Math.round(Screen.pixelDensity * 20)
+			color: "black"
+		}
+
 		Kirigami.Label {
 			Layout.columnSpan: 2
 			Layout.fillHeight: true