Add OSUtils for platform-specific functionality.

Moves MacUtils into a separate sub folder and adds
WinUtils, NixUtils, and OSUtils for platform-native
code on Windows, Unix-like, and generic/all systems.

src/gui/osutils/nixutils/NixUtils.h

@@ -0,0 +1,48 @@
+/*
+ * Copyright (C) 2020 KeePassXC Team <team@keepassxc.org>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 or (at your option)
+ * version 3 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef KEEPASSXC_NIXUTILS_H
+#define KEEPASSXC_NIXUTILS_H
+
+#include "gui/osutils/OSUtilsBase.h"
+#include <QPointer>
+
+class NixUtils : public OSUtilsBase
+{
+    Q_OBJECT
+
+public:
+    static NixUtils* instance();
+
+    bool isDarkMode() override;
+
+private:
+    explicit NixUtils(QObject* parent = nullptr);
+    ~NixUtils() override;
+
+private:
+    static QPointer<NixUtils> m_instance;
+
+    Q_DISABLE_COPY(NixUtils)
+};
+
+inline NixUtils* nixUtils()
+{
+    return NixUtils::instance();
+}
+
+#endif // KEEPASSXC_NIXUTILS_H