Extracted ContentManager::startDownload()

Extracted from ContentManager::downloadBook() the part that should be
executed asynchronously into a new function
`ContentManager::startDownload()`.

src/contentmanager.cpp

@@ -511,9 +511,23 @@ void ContentManager::downloadBook(const QString &id)
     kiwix::Book book = getRemoteOrLocalBook(id);
     const auto downloadPath = getSettingsManager()->getDownloadDir();
 
-    std::string downloadId;
     try {
         DownloadManager::checkThatBookCanBeDownloaded(book, downloadPath);
+    } catch ( const KiwixAppError& err ) {
+        showErrorBox(err, mp_view);
+        return;
+    }
+
+    startDownload(id);
+}
+
+void ContentManager::startDownload(QString id)
+{
+    kiwix::Book book = getRemoteOrLocalBook(id);
+    const auto downloadPath = getSettingsManager()->getDownloadDir();
+
+    std::string downloadId;
+    try {
         downloadId = DownloadManager::startDownload(book, downloadPath);
     } catch ( const KiwixAppError& err ) {
         showErrorBox(err, mp_view);

src/contentmanager.h

@@ -118,6 +118,7 @@ private: // functions
     const kiwix::Book& getRemoteOrLocalBook(const QString &id);
     QString getRemoteLibraryUrl() const;
 
+    void startDownload(QString bookId);
     void removeDownload(QString bookId);
     void downloadDisappeared(QString bookId);
     void downloadCompleted(QString bookId, QString path);