New features and one bugfix

[FEATURE] Show registration in saveRegistrationResultAction #1136

Note, that there is a new argument for the saveRegistrationResultAction. Existing route enhancers should be adapted as following (new argument reguid):

-
  routePath: '/save-registration-result/{eventuid}/{reguid}/{result}/{hmac}'
  _controller: 'Event::saveRegistrationResult'
  _arguments:
    eventuid: eventuid
    reguid: reguid
    result: result
    hmac: hmac

[FEATURE] Added PSR-14 event to stop cancellation of depending registrations #1146
[FEATURE] Added ProceedWithPaymentActionEvent PSR-14 event #1139

[BUGFIX] Respect DisableLanguageMenuProcessor in registration #1138
[TASK] PHP 7.4 compatibility for ProcessCancelDependingRegistrationsEvent #1146
[TASK] Added PHP 7.4 compatibility to ProceedWithPaymentActionEvent #1139
[DOCS] Add link to documentation about breaking changes #1142
[DOCS] Fixed wrong TypoScript reference