A major bugfix for extension registration, ext-coupon does now purge coupons:

A major bugfix for extension registration, ext-coupon does now purge coupons:
- Fixed registration of extensions with update/register dependencies, they
  were not executed before the actual selected extension was registered
- Purging of coupons added (was not yet implemented) with details of every purged coupon
- Fixed some language strings for link titles (again)
- TODOs.txt updated