Showing nested popups result in a Wayland protocol error for xdg shell

1. Start weston
2. Start the mainwindows example from qtbase with "env QT_WAYLAND_USE_XDG_SHELL=1 ./mainwindows -platform wayland"
3. Click the "Tool bars" menu
4. Try to expand one of the sub menus

The application terminates with a protocol error:

xdg_shell@17: error 2: xdg_popup was not created on the topmost popup

The problem is that we always use a toplevel surface as the xdg_popup parent, which is incorrect.