Add PromptDialog

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

Toga offers dialogs of a few different types, including an InfoDialog, QuestionDialog, ConfirmDialog, ErrorDialog and more.

But it does not offer a dialog that captures arbitrary user input, like a number or text.

### Describe the solution you'd like

Add a new dialog type, `PromptDialog`, that takes a specific type — like integer, float or string — and captures user input of that type.

Example:

The example above shows a custom dialog I made using Window, for my custom debugging toolset. But it would probably be better to have `Cancel` and `Ok` buttons like other dialogs.

### Describe alternatives you've considered

None

### Additional context

_No response_

Start by reading Toga's existing InfoDialog, QuestionDialog, ConfirmDialog, and ErrorDialog APIs, along with the Window usage mentioned in the issue. Define how PromptDialog should accept integer, float, or string input and how its Cancel and Ok results should behave. Done means a native dialog can capture the requested input type with the expected buttons and behavior across supported platforms.

Written by the indexing model from the issue text.