Thunar keyboard shortcuts are not initialized when opening preferences dialog from settings menu
## Version information
12e180ec0f8 on Arch Linux
## Steps to reproduce
1. Log in to a new session
1. From the Applications Menu, select Settings > Thunar Preferences
1. Click on the Shortcuts tab
1. Notice that the shortcuts are not initialized / listed properly
## Current behavior
The shortcuts are not initialized when opening the preferences dialog from the Settings menu. If I open the preferences dialog from within Thunar, the shortcuts are displayed properly.
## Expected outcome
The shortcuts are displayed properly regardless of how the preferences dialog is opened.
## Backtrace
Example Valgrind trace:
```
Conditional jump or move depends on uninitialised value(s)
at 0x4C53B2B: _gtk_accel_label_class_get_accelerator_label (gtkaccellabel.c:784)
by 0x4C52D31: gtk_accelerator_get_label (gtkaccelgroup.c:1712)
by 0x48DE769: xfce_shortcuts_editor_create_contents (xfce-shortcuts-editor.c:490)
by 0x48DF30A: xfce_shortcuts_editor_new_variadic (xfce-shortcuts-editor.c:266)
by 0x48DF3AF: xfce_shortcuts_editor_new (xfce-shortcuts-editor.c:181)
by 0x18CE71: thunar_preferences_dialog_init (thunar-preferences-dialog.c:1426)
by 0x56FC768: g_type_create_instance (gtype.c:1951)
by 0x56DEABA: g_object_constructor (gobject.c:2981)
by 0x48F8ED1: xfce_titled_dialog_constructor (xfce-titled-dialog.c:154)
by 0x56DECE0: g_object_new_with_custom_constructor (gobject.c:2524)
by 0x56DEF6A: g_object_new_internal (gobject.c:2604)
by 0x56E082D: g_object_new_with_properties (gobject.c:2769)
by 0x56E166E: g_object_new (gobject.c:2415)
by 0x18D671: thunar_preferences_dialog_new (thunar-preferences-dialog.c:1521)
by 0x1568D2: thunar_dbus_service_display_preferences_dialog (thunar-dbus-service.c:752)
by 0x137803: _g_dbus_codegen_marshal_BOOLEAN__OBJECT_STRING_STRING (thunar-dbus-service-infos.c:444)
by 0x137874: thunar_dbus_file_manager_method_marshal_display_preferences_dialog (thunar-dbus-service-infos.c:1978)
by 0x56D8D17: g_closure_invoke (gclosure.c:833)
by 0x56EBF3B: signal_emit_unlocked_R (gsignal.c:3887)
by 0x56F4C3C: signal_emitv_unlocked (gsignal.c:3211)
by 0x56F4C3C: g_signal_emitv (gsignal.c:3111)
by 0x13921D: _thunar_dbus_file_manager_skeleton_handle_method_call (thunar-dbus-service-infos.c:5120)
by 0x54D3863: g_dbus_interface_method_dispatch_helper (gdbusinterfaceskeleton.c:618)
by 0x54D3BE0: skeleton_intercept_handle_method_call (gdbusinterfaceskeleton.c:660)
by 0x54B9C1A: call_in_idle_cb (gdbusconnection.c:5458)
by 0x55D54C6: g_idle_dispatch (gmain.c:6247)
by 0x55D689B: g_main_dispatch (gmain.c:3361)
by 0x55D9270: g_main_context_dispatch_unlocked (gmain.c:4212)
by 0x55D9270: g_main_context_iterate_unlocked (gmain.c:4277)
by 0x55D983D: g_main_context_iteration (gmain.c:4342)
by 0x548E7AF: g_application_run (gapplication.c:2715)
by 0x13F9E8: main (main.c:86)
```
## Screenshot

issue