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/browser/BrowserService.cpp

@@ -39,7 +39,7 @@
 #include "gui/MainWindow.h"
 #include "gui/MessageBox.h"
 #ifdef Q_OS_MACOS
-#include "gui/macutils/MacUtils.h"
+#include "gui/osutils/macutils/MacUtils.h"
 #endif
 
 const QString BrowserService::KEEPASSXCBROWSER_NAME = QStringLiteral("KeePassXC-Browser Settings");