Add shortcut to toggle the 3D editor's camera preview

A Camera3D node still has to be selected to initially enable camera
preview, but another node can then be selected and the preview can
be disabled by pressing the shortcut key again.

editor/plugins/node_3d_editor_plugin.cpp

@@ -4290,6 +4290,7 @@ Node3DEditorViewport::Node3DEditorViewport(Node3DEditor *p_spatial_editor, Edito
 
 	preview_camera = memnew(CheckBox);
 	preview_camera->set_text(TTR("Preview"));
+	preview_camera->set_shortcut(ED_SHORTCUT("spatial_editor/toggle_camera_preview", TTR("Toggle Camera Preview"), KEY_MASK_CMD | KEY_P));
 	vbox->add_child(preview_camera);
 	preview_camera->set_h_size_flags(0);
 	preview_camera->hide();