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/debugger/console.h

@@ -0,0 +1,14 @@
+// Copyright 2018 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+namespace Debugger {
+
+/**
+ * Uses the WINAPI to hide or show the stderr console. This function is a placeholder until we can
+ * get a real qt logging window which would work for all platforms.
+ */
+void ToggleConsole();
+} // namespace Debugger