Add copy filepath to clipboard action

[Iann29]

Adds a new action that copies the saved file path to clipboard. Useful when you need to quickly reference where the screenshot was saved.

• New action: copy-filepath-to-clipboard
• Keybind: Ctrl+Alt+C
• Auto-saves if file wasn't saved yet
• Works with wl-copy and native GTK clipboard

Can be used in actions-on-enter, actions-on-escape, etc.

Tested on my system and works well. Let me know if you'd like any changes!

[RobertMueller2]

Nice idea, thanks! Might be a bit before we get round to reviewing this.

I'm wondering if this needs to be a dedicated action, perhaps we should just always copy the filename when saving to file? Does anyone see any reason why we wouldn't want to?

[40024]

If both are coupled, I think it would be best if there was a config to split them I personally can see myself saving to file and not wanting to update my clipboard

[RobertMueller2]

OK, maybe a separate action is the way to go.

@Iann29 I do have a few remarks:

1. I think the action should do just that: Copy the last used filename to clipboard, and if there is none, do nothing (or maybe log/notify there is none, I'd be fine with that). No auto-save. This should simplify this change a bit.
2. I think copy_text_to_external_process and save_to_external_process can be deduplicated. This requires a small change to handle_copy_clipboard, but that'd be fine. Can you try, please?
3. It would be great if you could add the new shortcut to README.md, along with <sup>NEXTRELEASE</sup> (The latter creates a dependency on #322)
4. Please use make fix to make the fmt task happy. Thanks :)