Logging: Add customizable logging backends and fmtlib based macros

* 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

src/core/settings.h

@@ -129,8 +129,6 @@ struct Values {
     float bg_green;
     float bg_blue;
 
-    std::string log_filter;
-
     // Audio
     std::string sink_id;
     bool enable_audio_stretching;
@@ -143,6 +141,7 @@ struct Values {
     // Debugging
     bool use_gdbstub;
     u16 gdbstub_port;
+    std::string log_filter;
 
     // Movie
     std::string movie_play;