This virtual method should be implemented by panel applets that wish to support Popover's natively.
As each Budgie Panel may house multiple GtkPopover widgets, each one must be registered with the manager.
During this call, it is safe to store a reference to the manager. In this call you should invoke
BudgiePopoverManager::register_popover to register your popover with the panel manager.
Each registered popover joins the global menu system of popovers in the panel. It is a requirement to register, otherwise the panel will not know when to expand and collapse the main panel harness to accommodate the GtkPopover.