Fix lock hierarchy violation in Process (lock ordering of ThreadList mutex and StackFrameList mutex)
- this fix ensures the ThreadList mutex is always locked before the StackFrameList mutex

Situation where deadlock could occur (without this fix):
Thread 1 is in Process::WillResume and locks the ThreadList mutex (on entry), and subsequently calls StackFrameList::Clear() which locks the StackFrameList mutex.
Meanwhile, thread 2 is in Process::RunThreadPlan and calls Thread::SetSelectedFrame() (which locks the StackFrameList mutex) before calling GetSelectedThread (which attempts to lock the ThreadList mutex)

In my testing on both Linux and Mac OS X, I was unable to reproduce any hangs with this patch applied.