Merge tag '2.0-beta1' into develop

Version 2.0-beta1
Require contao/core >=3.2.1,<=3.4-dev now.
Use more event name constants.
Add new generate item classes event.
Add new xnav_link_is_visible twig function.
Improve phpdoc.
Add new show-link/hide-link css classes.
Merge branch 'release/2.0-beta2'