Fix Frame Advance and FifoPlayer pause/unpause/stop.

CPU::EnableStepping is not atomic but is called from multiple threads
which races and leaves the system in a random state; also instruction
stepping was unstable, m_StepEvent had an almost random value because
of the dual purpose it served which could cause races where CPU::Run
would SingleStep when it was supposed to be sleeping.

FifoPlayer never FinishStateMove()d which was causing it to deadlock.
Rather than partially reimplementing CPU::Run, just use CPUCoreBase
and then call CPU::Run(). More DRY and less likely to have weird bugs
specific to the player (i.e the previous freezing on pause/stop).

Refactor PowerPC::state into CPU since it manages the state of the
CPU Thread which is controlled by CPU, not PowerPC. This simplifies
the architecture somewhat and eliminates races that can be caused by
calling PowerPC state functions directly instead of using CPU's
(because they bypassed the EnableStepping lock).

1) Apparently wxString::Format is type safe, and passing a u32 to it
with the format "%lu" crashes with a meaningless assertion failure.
Sure, it's the wrong type, but the error sure doesn't help...

2) "A MenuItem ID of Zero does not work under Mac".  Thanks for the
helpful assert message, no thanks for making your construct have random
platform-specific differences for no reason (it's not like menu item IDs
directly correspond to a part of Cocoa's menu API like they do on
Win32).

Incrementing the reference count here isn't necessary, as they construct with a count of 1. Incrementing again results in the attributes not being freed.

This technically also fixes a memory leak in WatchView.cpp, because the table setting was done such that the grid wouldn't take ownership of the table, which means said table wouldn't be deleted in the grid's destructor.