* Change the logging backend to support multiple sinks through the
Backend Interface
* Add a new set of logging macros to use fmtlib instead.
* Qt: Compile as GUI application on windows to make the console hidden by
default. Add filter configuration and a button to open log location.
* SDL: Migrate to the new logging macros
Add a case of `Log::Level::Count` to all switch statements that
dispatch on `Log::Level`. The case simply asserts `false` and notes
the invalid log level.
It provided a large increase in complexity of the logging system while
having a negligible performance impact: the usage patterns of the ring
buffer meant that each log contended with the logging thread, causing
it to effectively act as a synchronous extra buffering.
Also removed some broken code related to filtering of subclasses which
was broken since it was introduced. (Which means no one ever used that
feature anyway, since, 8 months later, no one ever complained.)
