Refactor logging code to get rid of a mutex.

After fixing the deadlock in #3006, it is now possible for log
messages to flood in faster than UpdateLog can render them.

This causes it to never return, locking up the gui thread and
filling the windows message queue (which triggers the stack
overflow bug in older versions of Wx)

Mostly so we can iterate over which LogListeners are enabled without
having to aquire a lock.

An KHR_debug callback could end up waiting for a mutex
on a thread which calls windows system functions.

While this is not expressly forbidden by the standards,
it does forbid directy calling windows system functions
from a KHR_debug callback. Close enough.

Replace it with the DebuggerLogListener.

Technically fixes a memory leak (which wouldn't matter because the timer is only created once and destroyed on shutdown).

Also starting and stopping the timer does not cause leaks.