Add visibility to constants

[gmponos]

PSR-12 Just for reference if it gets accepted:

Visibility MUST be declared on all constants if your project PHP minimum version supports constant visibilities (PHP 7.1 or later).

Personally I like it also this way better. Maybe you should revise some of them if they should be public but this would be a BC. Since you are moving to v2.0 it's a good chance.

[Seldaek]

Thanks, adjusted visibilities in bb99e4c