Text input dialog

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

I am porting a tkinter app to toga,
and had been using `tkinter.simpledialog.askstring`
to prompt for input,
and i could not find any equivalent in toga.

### Describe the solution you'd like

A dialog with a TextInput or a way to make custom dialogs.

### Describe alternatives you've considered

subclassing `toga.Window.factory.dialogs.ConfirmDialog`,
putting the `TextInput` and ok/cancel buttons in the main window

### Additional context

_No response_

Start by reading Toga's Window.factory.dialogs.ConfirmDialog and the TextInput widget, then compare them with tkinter.simpledialog.askstring. The issue is resolved when Toga supports a text-input dialog or a documented way to create custom dialogs with text input and OK/cancel actions.

Written by the indexing model from the issue text.