Improve handling of command line arguments (esp for document-based apps)

[freakboy3742]

Describe the bug

If an app declares document types, any command line argument that isn't a document will result in a Don't know how to open documents with extension ... error.

Steps to reproduce

Run the testbed with a test specifier:

briefcase run --test -- tests/app/test_app.py

results in an error because the testbed doesn't know how to handle documents of type .py.

Expected behavior

Functionally, Testbed shouldn't raise an error or warning for any "normal" operation.

More generally, it should be possible for a document-based app to handle arguments that aren't documents. It might be helpful for this list of arguments to be processed using standard argparse handling, with the argparse instance being configured using a standard interface (with a default implementation).

Screenshots

No response

Environment

• Operating System: Any desktop platform
• Python version: All
• Software versions:
  • Toga: 0.3.19+

Logs

Additional context

There may be an argument that this is a quirk of testbed - that testbed is a document-based app by necessity, but not representative of "real" document-based apps, and the API contract of document-based apps is "the command-line arguments can only be documents". This would be an extreme option, but not a completely indefensible one.

For GTK and Windows, this should be relatively straightforward - replace the sys.argv[] reference in App._create_initial_windows with something based on an argparse definition.

For cocoa it's a little more complex, because the document handling is managed by Cocoa (so as to enable "drop document onto app" handling). We either need to separate "first launch" argument handling from the application:openFiles: handler, or we need a strategy for how document names are handled in the context of a full list of arguments.

[HalfWhitt]

Even outside of the testbed, simply running a script (that launches an app) also generates such an error.

$ python test_script.py
test_script.py: Don't know how to open documents with extension .py

(Running it as a module with -m doesn't.)