cmake-gui: Enable High DPI scaling

This Qt flag enables UI scaling based on scale factor provided by the
operating system.  Only supported on versions of Qt >= 5.6.

The flag has no effect on macOS, where high DPI support is already
enabled as a setting in the bundle's info.plist.

Fixes: #17026

Source/QtDialog/CMakeSetup.cxx

@@ -92,6 +92,10 @@ int main(int argc, char** argv)
   cmAddPluginPath();
 #endif
 
+#if QT_VERSION >= 0x050600
+  QGuiApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
+#endif
+
   QApplication app(argc, argv);
 
   setlocale(LC_NUMERIC, "C");