Add settings API and specification

The details of the API are in the `/docs/specsSettings-API.md` file.

The enums do not list all available settings, more are to be added as
the settings API replaces inline QSettings calls.

src/settings/settings.cpp

@@ -0,0 +1,76 @@
+#include <utility>
+#include <QSettings>
+#include <easylogging++.h>
+#include "../overlaycontroller.h"
+#include "settings_controller.h"
+
+namespace settings
+{
+SettingsController& getSettingsController()
+{
+    static SettingsController s{};
+
+    return s;
+}
+
+void saveChangedSettings()
+{
+    getSettingsController().saveChangedSettings();
+}
+
+void saveAllSettings()
+{
+    LOG( INFO ) << "Saving all settings.";
+    getSettingsController().saveAllSettings();
+    LOG( INFO ) << "All settings saved.";
+}
+
+[[nodiscard]] bool getSetting( const BoolSetting setting )
+{
+    return getSettingsController().getSetting<bool>( setting );
+}
+
+void setSetting( const BoolSetting setting, const bool value )
+{
+    getSettingsController().setSetting( setting, value );
+}
+
+[[nodiscard]] double getSetting( const DoubleSetting setting )
+{
+    return getSettingsController().getSetting<double>( setting );
+}
+
+void setSetting( const DoubleSetting setting, const double value )
+{
+    getSettingsController().setSetting( setting, value );
+}
+
+[[nodiscard]] int getSetting( const IntSetting setting )
+{
+    return getSettingsController().getSetting<int>( setting );
+}
+
+void setSetting( const IntSetting setting, const int value )
+{
+    getSettingsController().setSetting( setting, value );
+}
+
+[[nodiscard]] std::string getSetting( const StringSetting setting )
+{
+    return getSettingsController().getSetting<std::string>( setting );
+}
+
+void setSetting( const StringSetting setting, const std::string value )
+{
+    getSettingsController().setSetting( setting, value );
+}
+
+std::string initializeAndGetSettingsPath()
+{
+    // The static object is initialized the first time the function is called.
+    // Having a dedicated function for this allows more control over when this
+    // happens.
+    return getSettingsController().getSettingsFileName();
+}
+
+} // namespace settings