Add support for monetary events

[jensschuppe]

• [x] Add price fields to form definition (also for additional participants; on registration only, disallow updating price options for now)
• [ ] Process selected price field options (re-implementing Core's event form processing spaghetti code - ideally by moving some of Core's functionality into the API)
• [ ] Allow for client-side improvements, e.g. live calculation of total amount for registration, including formatting with correct currency
• [ ] Prepare for remote payments