Add a developer option to duplicate tiles

This is useful for testing how the UI behaves with different numbers of participants.

public/locales/en-GB/app.json

@@ -132,6 +132,7 @@
     "developer_settings_label": "Developer Settings",
     "developer_settings_label_description": "Expose developer settings in the settings window.",
     "developer_tab_title": "Developer",
+    "duplicate_tiles_label": "Number of duplicate tiles",
     "feedback_tab_body": "If you are experiencing issues or simply would like to provide some feedback, please send us a short description below.",
     "feedback_tab_description_label": "Your feedback",
     "feedback_tab_h4": "Submit feedback",