Update copyright

[vkbo]

Summary:

This PR updates the copyright notices in the source, but changes how they are expressed so this won't need to be done each year except for in the two license files included in Debian and Windows releases.

• For source code files, only the year the file was created is used, if the code was new at the time.
• If the code in the file was moved there, the year the class object was created is used instead.
• There will no loner be a year range added, preventing the need for yearly updates.
• Additional copyright entries are added for major contributions to the file by other people than the main author. For minor or trivial changes, this is not added.

These copyright statements are pretty much redundant, but they are useful meta data about the code's history. If there at some point are a lot of contributors to files, the names may be replaced with "and contributors". This is a non-issue at this time.

Only @bkmeneguello and @Ryex have been added to the list in a handful of files.

Related Issue(s):

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed

[vkbo]

Decided to go with:

Copyright (C) 20xx Veronica Berglyd Olsen and novelWriter contributors

It avoids the issue of determining what is and isn't a significant contributions, and the contributors are already listed in both the CREDIT file and the About dialog.