Toast: add dismiss signal with dismissal reason

[danirabbit]

Always send the closed signal when Toasts are closed manually or automatically, except when the default action is triggered. Also, add a close reason in case we want to respond differently based on whether we closed manually, expired, or withdrawn.

Not sure if we need the close reason actually. afaict nothing is connecting to this signal anywhere. The motivation for this PR is that Desktop settings was watching for the toast revealer to be closed elementary/switchboard-plug-pantheon-shell@c031ada. Happy to exclude that if it seems unnecessary

[jeremypw]

Just a minor query otherwise looks good.

[tintou]

Note that adding a parameter to the "close" signal is an API break (meaning that all the code using it will break without being recompiled)

[davidmhewitt]

Note that adding a parameter to the "close" signal is an API break (meaning that all the code using it will break without being recompiled)

Yeah, in this case, it may be better to add a new signal (vanished maybe), and mark closed as deprecated. Because not only does it break the API contract, it's also a change in behaviour with the closed signal now being fired for other reasons when it wasn't previously.

[danirabbit]

Added a new signal dismissed, after https://valadoc.org/libadwaita-1/Adw.Toast.dismissed.html and deprecated the old signal