Update templated content after initial call to create

**Describe the bug**
After following the Beeware Tutorial, I decided to build a 2nd MSI Installer for Windows with a bump to the app version to 0.0.2. This created a file : `Hello World-0.0.2.msi` which I ran (I had already installed the previous `Hello World-0.0.1.msi`. When I look in the Control Panel\Programs\**Programs and Features** I see two instances of the app and both show version 0.0.1

**To Reproduce**
Steps to reproduce the behavior:

1. On a Windows PC, complete the Beeware Tutorial to the end of Tutorial 3.
https://docs.beeware.org/en/latest/tutorial/tutorial-3.html

2. Run the MSI created at the end of Tutorial 3 to install the application.
`Hello_World-0.0.1.msi`

3. Proceed with Tutorial 4 which updates the application code.

4. Bump the project version to 0.0.2 in `pyproject.toml`

```[tool.briefcase]
project_name = "Hello World"
bundle = "com.example"
version = "0.0.2"
```

5. Build a new MSI installer by running these commands :
```
briefcase update
briefcase package
```

6. Run the 2nd MSI created at install the application update.
`Hello_World-0.0.2.msi`

**Expected behavior**
I was expecting the 2nd MSI version to update the already installed version. While it does appear installed app code is updated (C:\Users\User\AppData\Local\Programs\Hello World\app\helloworld\app.py is the latest version), I don't think there should be multiple apps to uninstall in **Programs and Features**.

**Screenshots**
Attached

**Environment:**
- Operating System: Windows 10 Pro, Version 2004
- Python version: Python 3.7.7

- Software versions:
- Briefcase: 0.3.3
- Toga: 0.3.0.dev22

**Additional context**

RKM 2022-12-22: Although this was originally reported as a specific bug with Windows, it's a manifestation of a more generic problem - any content generated from template is locked in stone. We need a generic ability to update templated content after the original template has been generated, without the need to delete and re-generate the project.

Reproduce the Windows flow from the Beeware Tutorial through Tutorial 4, using pyproject.toml and the `briefcase update` and `briefcase package` commands. Trace how templated content is generated and retained after the initial `create` call. Done means template-derived content can be updated without deleting and regenerating the project, including avoiding duplicate installed entries.

Written by the indexing model from the issue text.