Ensure app name is normalised, and comes from an explicit source

[freakboy3742]

Makes 2 small changes to the determination of app name:

• If an App ID is provided (which is documented as required unless there's Briefcase-style packaging metadata), the app name is determined from the app ID, rather than the module name
• The app ID is always normalised.

This fixes #3926, which identified that running python -m toga_demo resulted in the wrong app icon (because the app icon was evaluated as resources/toga_demo, whereas if you ran with briefcase, you would get the icon (because Briefcase would embed the icon into the app binary)

It also fixes the fact that toga wasn't listed as a dependency (because of the use of setuptools-dynamic-dependencies), and that some of the other definitions needed for successful Briefcase operation weren't listed.

The app won't run on iOS, Android or Web, so those configurations are now disabled.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[mhsmith]

Let's keep discussion about the substance of the change in one place in #3926.

I don't know what's happening with the GTK testbed, but the Qt failures look like they were caused by the most recent changes.

[freakboy3742]

The GTK failure was transient; the Qt issue was caused because I forgot the testbed explicitly sets the app name (using the underscore variant) because of the code sharing between the two testbeds.