implementations should use interface values (vs raw)

[aerickson]

Describe the bug The code below explodes on iOS and Android (Mac/cocoa must do some casting?).

It works if the integers are cast via str().

        self.interval_input = toga.TextInput(
            placeholder=2, readonly=True, initial=2
        )

To Reproduce Steps to reproduce the behavior:

1. Create an app with the code above
2. briefcase run iOS or briefcase run android
3. App will crash

Expected behavior The code above should work.

Screenshots n/a

Environment:

• Operating System: Mac 11.4
• Python version: 3.8
• Software versions: briefcase 0.3.5 toga 0.3.0.dev27 toga-cocoa 0.3.0.dev27 toga-core 0.3.0.dev27

[aerickson]

somewhat related: Selection boxes will explode on iOS if items=a_dict.values() vs a list(). Using items=a_dict.values() works fine on Android and Cocoa.

toga_iOS/widgets/selection.py", line 19, in pickerView_titleForRow_forComponent_

[freakboy3742]

This will be resolved (and tested) by the textinput widget audit in #1944.