mix IDE M2

Integrated debugger;
Hit F5 and you can step forwards/backwards through execution.

Indeed. These methods name could be called TransactionBuilder::getTransaction(...) and they would be exactly like Transaction(...); so, they add a layer of abstraction without adding functionality. 
Is this a WIP (work in progress) and the Transaction Builder will have more features?

Good to only have one way to do something, in the simplest way possible -- even if it's the same method name with different arguments? :)

If this is included then unit tests should be added and maybe consider move it to QEthereum as well.