Registers a pane, or replaces the one already under that id.
Idempotent by id, because a module may re-register across a destroy/recreate cycle and the second registration must not double the tab.
The pane to offer to the hosts.
Registers a pane, or replaces the one already under that id.
Idempotent by id, because a module may re-register across a destroy/recreate cycle and the second registration must not double the tab.