Wizards: Add placeholder text

... to class name field in "New Class" wizard.
This way, users understand right away that the class name can include
namespaces.

Change-Id: I84aaf2aca532a97ba40f72fd2385fcab40788970
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

share/qtcreator/templates/wizards/classes/cpp/wizard.json

@@ -36,6 +36,7 @@
                     "mandatory": true,
                     "type": "LineEdit",
                     "data": {
+                        "trPlaceholder": "Fully qualified name, including namespaces",
                         "validator": "(?:(?:[a-zA-Z_][a-zA-Z_0-9]*::)*[a-zA-Z_][a-zA-Z_0-9]*|)",
                         "completion": "namespaces"
                     }