3.0.0-rc.1

This is a release candidate for CSFML 3, which brings it in line with SFML 3 and allows for properly updating SFML.Net.

Beyond small fixes here and there the main changes in this release are:

• Updated to use SFML 3
• clang-tidy support added
• Many new tests added

Full Changelog: 2.6.1...3.0.0-rc.1