kcms/nightcolor: Use sensible precision for coordinates

Before, NumberField displayed 6 decimal digits for the coordinates, which 
implies centimeter-scale resolution, which is of course not the case.

Now it shows 2 decimal digits, which is reasonable and consistent with the
level of precision shown in Automatic mode.

kcms/nightcolor/package/contents/ui/NumberField.qml

@@ -15,10 +15,10 @@ QQC2.TextField {
 
     inputMethodHints: Qt.ImhFormattedNumbersOnly
 
-    text: backend
+    text: Math.round(backend * 100)/100;
 
     onBackendChanged: {
-        text = backend;
+        text = Math.round(backend * 100)/100;
     }
 
     onTextChanged: {