Launch Marketplace via the app icon instead of via Javascript

This is an alternative to PR #10. The code is much simpler, but it will take a bit longer to execute as it has to scroll through homescreen pages.

I also took this opportunity to do a bit of refactoring with the Marketplace app name, which was unnecessarily being set all over the place. It can still be overridden in an individual test, but doesn't need to be set for every test.

Tbh, this is probably a better approach than PR #10.