`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.
As of Adwaita 1.5.0, the GTK Box is not being properly unref'd when the
parent window is closed. Update the conditional to account for this.
Also add a couple of missing unref()s in errdefers.
Using `gtk4-layer-shell` still seems like the path of least resistance,
and to my delight it pretty much Just Works. Hurrah!
This implementation could do with some further polish (e.g. animations,
which can be implemented via libadwaita's animations API, and global
shortcuts), but as a MVP it works well enough.
It even supports tabs!
Fixes#4624.
