Merge pull request SFML#2080 from SFML/feature/backmerge

2.6.x -> master backmerge

Updated the changelog and version number for 2.5.1

Updated version number to SFML 2.5.0

Increased SFML's version number and updated the changelogs.

Incremented SFML version number and added changes to the changelog.

Fixed PrimitiveType documentation

Increased version to 2.3.2 and add changes to changelog.

Increased version to 2.3.1.

Increased version to 2.3, updated the changelog and license and added…

… Mario to the readme.txt.

Updated doc version and removed trailing spaces