Add placeholder string to ContactGroupEditorPage

Signed-off-by: Claudio Cambra <claudio.cambra@gmail.com>

src/contacts/qml/private/ContactGroupEditorPage.qml

@@ -78,6 +78,7 @@ Kirigami.ScrollablePage {
                 Kirigami.FormData.label: i18n("Name:")
                 text: contactGroupEditor.name
                 onTextChanged: contactGroupEditor.name = text;
+                placeholderText: i18n("Contact group name")
             }
 
             ColumnLayout {
@@ -205,4 +206,4 @@ Kirigami.ScrollablePage {
             }
         }
     }
-}
\ No newline at end of file
+}