Add widget as table element

## Expected Behavior
I want to add a table with Toga that one of its columns is a column of switches (checkboxes).
When I run the application, instead of putting the checkboxes in the desired column, it writes its representation string.
Is there any way you can fix that?

Example code:
```python
data = [
["a", "b", c"],
["d", "e", f"]
["g", "h", i"]
]

table = toga.Table(
headings=["choose", "1", "2", "3"]
style=Pack(flex=1),
)

for record in data:
checkbox = toga.Switch(label="label")
table.data.append(checkbox, *record)
main_box.add(table)
main_window.content = main_box
```

## Your Environment

* Python Version 3.7

* Operating System and Version

- [ ] macOS - version:
- [ ] Linux - distro: - version:
- [X] Windows - version:
- [ ] Other - name: - version:

* Toga Version 0.3.0.dev19

* Toga Target (the type of app you are trying to generate)

- [ ] android
- [ ] cocoa
- [ ] django
- [ ] gtk
- [ ] iOS
- [ ] tvOS
- [ ] watchOS
- [ ] winforms
- [X] win32
- [ ] Other (please specify)

Start by reproducing the provided Toga Table and Switch example on the Windows win32 target, then trace how table cell values are rendered. Done means a Switch supplied as a table cell appears as a checkbox in its column instead of being converted to its representation string.

Written by the indexing model from the issue text.