Event about refund payment generation + test application structure change

Improvements:

• services.xml loaded in DI extension
• knp_snappy configuration removed from config.yml (as plugin supports SymfonyFlex)
• Refund payment methods resolved with order channel
• RefundPaymentGenerated event thrown after RefundPayment generation

Bug fixes:

• Credit memo sorting finally fixed

Tests:

• Symfony4 catalog structure introduced to test application