Default to org.kde.desktop QQC2 style

plasma-integration does that for us, but that obviously doesn't work for non-Plasma desktops.

src/main.cpp

@@ -58,6 +58,10 @@ int main(int argc, char *argv[])
     QQuickStyle::setStyle(QStringLiteral("Material"));
 #else
     QApplication app(argc, argv);
+    // Default to org.kde.desktop style unless the user forces another style
+    if (qEnvironmentVariableIsEmpty("QT_QUICK_CONTROLS_STYLE")) {
+        QQuickStyle::setStyle(QStringLiteral("org.kde.desktop"));
+    }
 #endif
 
     QApplication::setOrganizationName("KDE");