ContentManagerModel::Downloads became a class

... only one small step away from being thread-safe

src/contentmanager.cpp

@@ -543,7 +543,7 @@ void ContentManager::downloadBook(const QString &id, QModelIndex index)
         downloadBook(id);
         auto node = getSharedPointer(static_cast<RowNode*>(index.internalPointer()));
         const auto newDownload = std::make_shared<DownloadState>();
-        m_downloads[id] = newDownload;
+        m_downloads.set(id, newDownload);
         node->setDownloadState(newDownload);
     }
     catch ( const ContentManagerError& err )

src/contentmanagermodel.h

@@ -22,7 +22,33 @@ public: // types
     typedef QList<BookInfo>         BookInfoList;
 
     // BookId -> DownloadState map
-    typedef QMap<QString, std::shared_ptr<DownloadState>> Downloads;
+    class Downloads
+    {
+    private:
+        typedef std::shared_ptr<DownloadState> DownloadStatePtr;
+        typedef QMap<QString, DownloadStatePtr> ImplType;
+
+    public:
+        void set(const QString& id, DownloadStatePtr d) {
+            impl[id] = d;
+        }
+
+        DownloadStatePtr value(const QString& id) const {
+            return impl.value(id);
+        }
+
+        QList<QString> keys() const {
+            return impl.keys();
+        }
+
+        void remove(const QString& id) {
+            impl.remove(id);
+        }
+
+    private:
+        ImplType impl;
+    };
+
 
 public: // functions
     explicit ContentManagerModel(QObject *parent = nullptr);