New logging system

This PR revamps the logging system, re-writing it entirely. The code has pretty extensive documentation (I hope) but here are the highlights:
 - Multi-threaded logging, avoiding excessive logging from slowing down the emulation as much. (Especially on Windows where console writes are slow.)
 - Nicely trimmed paths relative to the source directory, without the source repository prefix.
 - Completely re-organized severity levels and log classes, with documentation providing guidelines for use of each class which should prevent misuse.
 - All the logging statements in the code have been updated to follow the new guidelines.
 - Runtime-configurable filter (currently read from the configuration file) allowing configuration of the log levels for each different log class.
 - Possibility of future extensions to support logging to the GUI or silencing of spammy log messages. (Planned for future PRs.)
Well, this would require moving the entire log system into core. I think Common should turn into something that contains infrastructural stuff, even if it's not generic, and Core should have just the stuff directly related to emulation.