add support for platform-dependent styles

This will allow us to work around platform-dependent quirks
in styling. For 2.1 this is needed to work around the issue of
the checkmark pushing text to the right and getting cut off
in WEffectSelector only on macOS.

src/skin/legacyskinparser.cpp

@@ -1873,6 +1873,26 @@ QString LegacySkinParser::getStyleFromNode(const QDomNode& node) {
                                            fileBytes.length());
         }
 
+        QString platformSpecificAttribute;
+#if defined(Q_OS_MAC)
+        platformSpecificAttribute = "src-mac";
+#elif defined(__WINDOWS__)
+        platformSpecificAttribute = "src-windows";
+#else
+        platformSpecificAttribute = "src-linux";
+#endif
+
+        if (styleElement.hasAttribute(platformSpecificAttribute)) {
+            QString platformSpecificSrc = styleElement.attribute(platformSpecificAttribute);
+            QFile platformSpecificFile(platformSpecificSrc);
+            if (platformSpecificFile.open(QIODevice::ReadOnly)) {
+                QByteArray fileBytes = platformSpecificFile.readAll();
+
+                style += QString::fromLocal8Bit(fileBytes.constData(),
+                                                fileBytes.length());
+            }
+        }
+
 // This section can be enabled on demand. It is useful to tweak
 // pixel sized values for different scalings. But we should know if this is
 // actually used when migrating to Qt5