Add folder dropdown icons to VDS (#5632)

* add svg

* update pixmap cache

* get icon to work

* hide icon when not clickable

* use consistent naming

* use expandOnly because apparently that leads to higher image quality

cockatrice/src/client/ui/pixel_map_generator.h

@@ -106,6 +106,19 @@ public:
     }
 };
 
+class DropdownIconPixmapGenerator
+{
+private:
+    static QMap<QString, QPixmap> pmCache;
+
+public:
+    static QPixmap generatePixmap(int height, bool expanded);
+    static void clear()
+    {
+        pmCache.clear();
+    }
+};
+
 QPixmap loadColorAdjustedPixmap(const QString &name);
 
 #endif