Emitted when a tab should be transferred into a new window.
This can happen after a tab has been dropped on desktop.
The signal handler is expected to create a new window, position it as
needed and return its
AdwTabView that the page will be transferred into.
The default handler is called after the handlers added via
AdwTabView from the new window.
|The data is owned by the called function.
|The return value can be