cmake-gui: Use icon from system theme if available

Source/QtDialog/CMakeSetup.cxx

@@ -146,7 +146,7 @@ int main(int argc, char** argv)
   QIcon appIcon;
   appIcon.addFile(":/Icons/CMakeSetup32.png");
   appIcon.addFile(":/Icons/CMakeSetup128.png");
-  QApplication::setWindowIcon(appIcon);
+  QApplication::setWindowIcon(QIcon::fromTheme("cmake-gui", appIcon));
 
   CMakeSetupDialog dialog;
   dialog.show();