adw-toast: Option for "clipboard-paste"

[gpanders]

The adw-toast option in #4185 has a clipboard-copy option to show a notification when text was copied to the clipboard.

We should also add a clipboard-paste option to show a notification when text was read from the clipboard. This is especially useful when users use clipboard-read = allow so that they know when an application is reading from their clipboard with OSC 52.