xdg-shell/popup: Allow custom parent by passing null as parent

Allow using some other protocol (custom, or future xdg_* based) to set
up the parent-child relationship of a popup. This allows future
protocols to use xdg_popup when mapping popups over surfaces not based
on xdg_surface.

An example use case for this is the window menu, where a shells UI
client can use xdg_popup to create popup menus over windows it does not
have a xdg_surface of by having a custom protocol setting up the proper
parent-child relationship.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-By: Mike Blumenkrantz <zmike@osg.samsung.com>