EchidnaWindow.get_current_tab() can now be used independently.
This will help with reusing it in another places. I think it will be used a lot.
Additionally, it will return Result<A, &str> instead of panicking, as it's a utils function now and should never panic.
The MPL is a file-based license, rather than a codebase-based license.
Each files need to be marked as MPL.
If we don't do mark them, they may cause confusion.
Before, all the traits are implemented for EchidnaApplication.
This is added to better support using the app with many windows open.
EchidnaApplication may be obsolete now, but I'm not removing it for now.