Switch to sys_category

This release contains breaking changes, make sure to read the change log below

[!!!][TASK] Refactor category system #312

This release contains a major breaking change, since the local category system has been removed in favour to TYPO3 system categories. A category migration process will fully automate the migration of existing categories to TYPO3 system categories. Please refer to the extension manual (section Breaking changes) for further instructions.

[!!!][TASK] Change ObjectType to FileReference where meaningful #308

If you use the image field for the organisator record, make sure to access the image using event.organisator.image.uid.