Add backend capabilities

[patacca]

Add a property for the backend loaders to advertise the supported capabilities.
This is useful to catch immediately whether a registered feature has all the information needed from the backend to correctly work. An example of such a feature would be #63 that uses pcode that is not part of the official API

[RobinDavid]

Ok great! I hope the mechanism is not overly complex for future maintenance.

Just two small remarks:

• there is no capability for instruction group, do we add one ? (it is a capability of a backend in the end..)
• the newly created exception is never caught in the main qbindiff cli executable, which means using the tool can result in a Python stack trace. It would be nice to catch it and to exit nicely.

[patacca]

* there is no capability for instruction group, do we add one ? (it is a capability of a backend in the end..)

Indeed I thought about it, right now the behavior is that instruction groups are mandatory and we have a enum InstructionGroups that all backend have to align to. We could make them optional and add them as a capability but we should still need to rely on the unified InstructionGroups, unless we want to have different capabilities for different group types (like CAP_GROUP_CAPSTONE, CAP_GROUP_IDA, ...) but I feel like it's not a good decision.
In the end I am in favor of making instruction groups optional as, right now, the IDA backend currently doesn't offer them.

* the newly created exception is never caught in the main qbindiff cli executable, which means using the tool can result in a Python stack trace. It would be nice to catch it and to exit nicely.

You are right, I am gonna fix this

[patacca]

I've chenged a few things:

• Added the try/catch clause in the CLI to gracefully exiting in case of an unsatisfied capability
• Add PCODE and INSTR_GROUP capabilities. For now quokka supports both, binexport only INSTR_GROUP and ida none.
• Adjusted the features that were affected by those capabilities
• Rewrote the instruction group feature to address the recent changes in Fix the JumpNb feature #61
• The PR passes the tests