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/common/logging/backend.h

@@ -6,8 +6,11 @@
 
 #include <chrono>
 #include <cstdarg>
+#include <memory>
 #include <string>
 #include <utility>
+#include "common/file_util.h"
+#include "common/logging/filter.h"
 #include "common/logging/log.h"
 
 namespace Log {
@@ -34,6 +37,80 @@ struct Entry {
     Entry& operator=(const Entry& o) = default;
 };
 
+/**
+ * Interface for logging backends. As loggers can be created and removed at runtime, this can be
+ * used by a frontend for adding a custom logging backend as needed
+ */
+class Backend {
+public:
+    virtual ~Backend() = default;
+    virtual void SetFilter(const Filter& new_filter) {
+        filter = new_filter;
+    }
+    virtual const char* GetName() const = 0;
+    virtual void Write(const Entry& entry) = 0;
+
+private:
+    Filter filter;
+};
+
+/**
+ * Backend that writes to stderr without any color commands
+ */
+class ConsoleBackend : public Backend {
+public:
+    static const char* Name() {
+        return "console";
+    }
+    const char* GetName() const override {
+        return Name();
+    }
+    void Write(const Entry& entry) override;
+};
+
+/**
+ * Backend that writes to stderr and with color
+ */
+class ColorConsoleBackend : public Backend {
+public:
+    static const char* Name() {
+        return "color_console";
+    }
+
+    const char* GetName() const override {
+        return Name();
+    }
+    void Write(const Entry& entry) override;
+};
+
+/**
+ * Backend that writes to a file passed into the constructor
+ */
+class FileBackend : public Backend {
+public:
+    explicit FileBackend(const std::string& filename);
+
+    static const char* Name() {
+        return "file";
+    }
+
+    const char* GetName() const override {
+        return Name();
+    }
+
+    void Write(const Entry& entry) override;
+
+private:
+    FileUtil::IOFile file;
+    size_t bytes_written;
+};
+
+void AddBackend(std::unique_ptr<Backend> backend);
+
+void RemoveBackend(const std::string& backend_name);
+
+Backend* GetBackend(const std::string& backend_name);
+
 /**
  * Returns the name of the passed log class as a C-string. Subclasses are separated by periods
  * instead of underscores as in the enumeration.
@@ -49,5 +126,10 @@ const char* GetLevelName(Level log_level);
 Entry CreateEntry(Class log_class, Level log_level, const char* filename, unsigned int line_nr,
                   const char* function, std::string message);
 
-void SetFilter(Filter* filter);
+/**
+ * The global filter will prevent any messages from even being processed if they are filtered. Each
+ * backend can have a filter, but if the level is lower than the global filter, the backend will
+ * never get the message
+ */
+void SetGlobalFilter(const Filter& filter);
 } // namespace Log