Dialogs customization

### What is the problem or limitation you are having?

I need a second custom button in a Dialog, for example, in "About" I may need the "Okay", but also the "Copy" button, so that I can easily copy some version-related infos in a simple way... Or, anyway, it's normal to have buttons in Dialogues, more than one, with custom labels/actions.

### Describe the solution you'd like

make a Custom Dialog, where you can add buttons with labels and actions

### Describe alternatives you've considered

creating a window...? not so clean

### Additional context

_No response_

The issue does not name files, tests, or an entry point. Start by locating Toga's existing Dialog API and its native backend implementations, then compare how button labels and actions are currently represented; the work is done when a custom dialog can expose multiple caller-defined buttons with actions consistently across supported backends.

Written by the indexing model from the issue text.