This admittedly does a couple of refactors / changes to the Forwarding interfaces from #583:

• Moves the old base IForwarder interface to be IAbstractInterface, so we can reserve IForwarder as the "normal" (most basic and most commonly used) forwarding type
• Adds the IForwarderFee interface to the payable interfaces
• Updates the forwarding types to be an enum (encoded as uint8 I think?), with just two types
  • We can detect payable simply by the forwardFee() method returning EtherTokenConstant
• Maintains the isForwarder() interface for backwards compatibility with aragonOS 4
• Adds some tests for behaviour

May be easiest to review by just looking at the branch's code.

Maintains the isForwarder() interface for backwards compatibility with aragonOS 4

This is somewhat debatable, but I wanted to keep this because:

• It allows off-chain tooling to continue accessing isForwarder(), and in the majority of the case (e.g. IForwarder), this will continue to just work without any additional checks
• It allows updated tooling to do both the isForwarder() and forwarderType() checks, and if isForwarder() is true but forwarderType() returns 0, know they're dealing with an older aragonOS 4 contract (which only has one forwarding interface).