[RFC] Improve `spawnDialog` or add a new function

Problem

In the current implementation spawnDialog is designed to create a one-time "prompt" dialog:

• It doesn't provide control over the mounted dialog, for example, to unmount it or call its public methods
• Use case: https://github.com/nextcloud-libraries/nextcloud-dialogs/blob/main/lib/dialogs.ts#L80

But at the same time, it isn't convenient as a prompt, requiring to pass callback instead of returning a promise.

Proposals

1. Instead of only calling callback - return promise
   • How to determine if it was closed or unmounted?
2. Provide 2 functions:
   • One for one-time prompts returning a promise
   • One for mounting a dialog instance
3. Return promise with additional properties

Additional problem

In the current API props argument is passed as dialog component props, but it includes container and appContext, which are not dialog props.

We can mix it and only fix the implementation, or separate dialog's props and spawnDialog's options (breaking change).