Panel dropdown: support multiple plugins extending the same core dropdown

A (plugin) API that allows you to add buttons/jobs to this dropdown menu, taking inspiration from the Janitor API. In addition to the page itself, they should also be added to the dropdown menu in Pages sections.

Example use cases I explored/did in the past:

• DreamForm: clear form caches for accessing third-party APIs/clear form submissions/export form submissions as CSV
• Sync translations with third-party API (like Memsource/Phrase)

This already exists: https://getkirby.com/docs/reference/plugins/extensions/panel-dropdowns#extending-dropdowns

However, it currently has issues whenn multiple plugis want to extend the same core dropdown.

Is it possible to put an action there without a dialog before?

They will with v4.3:
https://github.com/getkirby/kirby/pull/6411
https://feedback.getkirby.com/638