CustomDialogs example: add Apply, Reset and Restore Defaults buttons

A custom dialog might have a use for these.

Change-Id: Ibef05dd63d65476e1225477d2e7ed9d24fbba9ed
Reviewed-by: Liang Qi <liang.qi@theqtcompany.com>

examples/quick/dialogs/systemdialogs/CustomDialogs.qml

@@ -262,6 +262,24 @@ Item {
                 onCheckedChanged: parent.updateButtons(button, checked)
             }
 
+            CheckBox {
+                text: "Apply"
+                property int button: StandardButton.Apply
+                onCheckedChanged: parent.updateButtons(button, checked)
+            }
+
+            CheckBox {
+                text: "Reset"
+                property int button: StandardButton.Reset
+                onCheckedChanged: parent.updateButtons(button, checked)
+            }
+
+            CheckBox {
+                text: "Restore Defaults"
+                property int button: StandardButton.RestoreDefaults
+                onCheckedChanged: parent.updateButtons(button, checked)
+            }
+
             CheckBox {
                 text: "OK"
                 checked: true