Improvements and Bugfixes

• [TASK] Performances are now displayed in Backend with a label build from their localized date and the event title. (Dirk Wenzel)
• [TASK] domain model Event extended by field relatedSchedules (Dirk Wenzel)
• [TASK] - new application icons (Timo Gockel)
• [BUGFIX] Add missing header schedules label (Steffen Mevius)
• [BUGFIX] invalid return values fixed (Dirk Wenzel)
• [BUGFIX] fixed error caused by missing SettingsInterface in PeriodConstraintRepositoryTrait (Dirk Wenzel)
• [BUGFIX] fixed error caused by invalid use statement (Dirk Wenzel)
• [TASK] obsolete updater MigrateTaskRecords removed (Dirk Wenzel)