Fix shift clicking on Add Sun/Environment to Scene buttons

(cherry picked from commit 1b6af3acd380358eda46be60d0278bca3ba39a2c)

editor/plugins/node_3d_editor_plugin.cpp

@@ -7788,6 +7788,9 @@ void Node3DEditor::_sun_environ_settings_pressed() {
 	sun_environ_popup->set_position(pos - Vector2(sun_environ_popup->get_contents_minimum_size().width / 2, 0));
 	sun_environ_popup->reset_size();
 	sun_environ_popup->popup();
+	// Grabbing the focus is required for Shift modifier checking to be functional
+	// (when the Add sun/environment buttons are pressed).
+	sun_environ_popup->grab_focus();
 }
 
 void Node3DEditor::_add_sun_to_scene(bool p_already_added_environment) {