SVC: Implemented GetThreadId.

For now threads are using their Handle value as their Id, it should not really cause any problems because Handle values are unique in Citra, but it should be changed. I left a ToDo there because this is not correct behavior as per hardware.
Threads: Implemented a sequential thread id
Threads: Remove a redundant function.

Use the next_thread_id variable directly.
Merge pull request #250 from Subv/cbranch_2

SVC: Implemented GetThreadId.