Modify handling of travertino dependency in example apps, CI and testbed.

[freakboy3742]

• Modifies the CI configuration so that Travertino is installed by pip finding the wheel in the dist folder, rather than by explicitly installing the package from the source directory.
• Modifies the testbed to look in the dist folder for prerelease wheels, but doesn't modify the platform configurations to use those wheels. This is so that local testing continues to work. However, the Textual backend will use the dist folder to install packages, so there is at least some testing of wheel usage by Briefcase.
• Updates all the examples and the demo app to include an explicit install of Travertino.
• Includes a hard version pin of PyGObject 3.50.0 to work around PyGObject>=3.51.0 depends on libgirepository 2.0 #3143.
• Includes a conditional coverage tweak that I noticed being reported on each of the standalone core test runs. This wasn't an issue for overall coverage, but it helps to keep local single-Python version test results clean.

Fixes #3154.
Refs #3143.

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