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/citra_qt/ui_settings.h

@@ -67,6 +67,9 @@ struct Values {
     QString room_port;
     uint host_type;
     qulonglong game_id;
+
+    // logging
+    bool show_console;
 };
 
 extern Values values;