Closes #628

Description

Intro on copyright for free software projects:
https://www.softwarefreedom.org/resources/2012/ManagingCopyrightInformation.html
https://opensource.com/law/14/2/copyright-statements-source-files

Note the following points:

• If N people contribute to a file, are we supposed to have N copyright statements in the file? This doesn't scale over time. Imagine what these files will look like 10 years from now, and fix the problem now.
• Having author names in a file encourages people to contribute for the wrong reasons. (Note: Contribute for better software instead of contribute for fame and money)
• Git keeps track of who contributed what changes. It's not necessary to have explicit copyright statements.

At the Apache Software foundation we discourage the use of author tags in source code. There are various reasons for this, apart from the legal ramifications. Collaborative development is about working on projects as a group and caring for the project as a group. Giving credit is good, and should be done, but in a way that does not allow for false attribution, even by implication. There is no clear line for when to add or remove an author tag. Do you add your name when you change a comment? When you put in a one-line fix? Do you remove other author tags when you refactor the code and it looks 95% different? What do you do about people who go about touching every file, changing just enough to make the virtual author tag quota, so that their name will be everywhere?

With those note in minds, what I've done here is pretty simple:

• Compare with meld or a similar tool the keepassx and keepassxc repo folder
• For every .cpp file that has difference and has been modified after the fork:
  • If there are substantial changes, add the new copyright notice.
  • If the .cpp file comes with an .h file, add the new copyright to that as well
  • If there are minor changes don't touch the file. (Useless adding copyright to that simple change like Q_EMIT to emit conversions)
• For every .cpp file that has a copyright notice and was added after the fork, maintain the copyright and add KeePassXC Team (we can't really remove the copyright without asking the author)
• .ui files don't need copyright notice
• For every CMakeLists.txt, if the file has some code (it's >10 lines long) add the copyright notice
• For every .png file, add the copyright notice to the COPYING file
• For every external/library code, add the copyright notice to the COPYING file

Other then this, I have to report some other things:

• Fixed the COPYING file, it was reporting KeePassX information/copyright
• Added a Comment in the COPYING file explaining what KeePassXC Team means. Some team members' email are missing.
• Removed from the zxcvbn copyright notice in COPYING the KeePassXC name (we have no copyright on that)
• Removed from src/cli/EntropyMeter.cpp the zxcvbn copyright since it was rewritten by me and (mostly by) @louib
• The following files were reporting Felix Geyer copyright but they were added after the fork by different people, I've changed the copyright to KeePassXC Team
  • src/cli/Extract.cpp
  • src/cli/Merge.cpp
  • src/core/PassphraseGenerator.cpp
  • src/core/PassphraseGenerator.h
  • src/gui/CloneDialog.cpp
  • src/gui/CloneDialog.h
  • src/http/HttpPasswordGeneratorWidget.cpp
  • src/keys/YkChallengeResponseKey.h
• All KeePassHTTP files were reporting the copyright in a different format, changed the format and added KeePassXC Team notice
• Copyright for share/icons/application/16x16/actions/group-empty-trash.png is missing, like noted here.

I hope I've done a fine work. Waiting for feedback on this. @keepassxreboot/core-developers

Checklist:

• ✅ I have read the CONTRIBUTING document. [REQUIRED]
• ✅ My code follows the code style of this project. [REQUIRED]
• ✅ All new and existing tests passed. [REQUIRED]