macOS: clipboard copy/paste notifications

[gpanders]

The GTK+Adwaita apprt supports showing a notification when text is copied to the clipboard. This discussion also proposes extending that for when text is pasted from the clipboard.

As I mentioned in that discussion, this is particularly useful to have so that a user knows when text is being read from the clipboard via OSC 52 (if clipboard-read is set to allow then the user currently has no way of knowing when this happens).

macOS does not currently support notifications when clipboard contents are copied or pasted (other than the usual "paste protection" and OSC 52 prompts). This discussion is a proposal to add a similar feature to the macOS apprt.

It doesn't necessarily need to be a toast window (which is not a common UX pattern in macOS AFAIK). Other prior art includes iTerm2 which shows a small dropdown bar underneath the tab bar.