`TabView` assumes to be the sole owner of all `Tab`s within a Window. As
such, it could close the managed `Window` once all tabs are removed from
its widget.
However, during `AdwTabView::close-page` signal triggered by libadwaita,
the `Tab` to be closed will gain an another reference for the duration
of the signal, breaking `TabView.closeTab` (called via
`Tab.closeWithConfirmation`) assumptions that having no tabs meant they
are all destroyed.
This commit solves the issue by scheduling `Tab.closeWithConfirmation`
to be run after `AdwTabView::close-page` signal has finished processing.
Fixes#7426
5edf0dffda