Sort Table Columns

## Expected Behavior
When you click on a column header, it should sort on the column by ascending/descending.

## Current Behavior
You can click on a column header, but nothing happens.

## Steps to reproduce
1. Create an app with a table

2. Populate the table

3. Run the app

4. Click on a column header

## Your Environment

* Python Version (list the specific version number): 3.7.6

* Operating System and Version (select from the following and list the specific version number; if your OS is not listed, list that as well)

- [x] macOS - version: 10.15.5
- [ ] Linux - distro: - version:
- [ ] Windows - version:
- [ ] Other - name: - version:

* Toga Version (list the specific version number or git hash)
0.2.15
* Toga Target (the type of app you are trying to generate)

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

No file or test is named in the issue. Start by reproducing the behavior in a Toga table app on the Cocoa target, then locate the table column-header handling; done means clicking a header sorts the displayed rows in ascending and descending order.

Written by the indexing model from the issue text.