Changed how thumbnails of local books are obtained

Before this change, thumbnails of local books were obtained by
`ContentManagerModel` (via ugly calls to
`KiwixApp::instance()->getLibrary()`). Now that task is
transferred to `ContentManager::updateModel()` - the thumbnail data, if
available, is loaded by `ContentManager::getBookInfos()` and passed to
`ContentManagerModel::setBooksData()` through a new entry `favicon`.

src/contentmanager.cpp

@@ -140,7 +140,7 @@ void ContentManager::updateModel()
 {
     const auto bookIds = getBookIds();
     BookInfoList bookList;
-    QStringList keys = {"title", "tags", "date", "id", "size", "description", "faviconUrl"};
+    QStringList keys = {"title", "tags", "date", "id", "size", "description", "faviconUrl", "favicon"};
     QIcon bookIcon;
     for (auto bookId : bookIds) {
         auto mp = getBookInfos(bookId, keys);
@@ -294,6 +294,28 @@ QString getFaviconUrl(const kiwix::Book& b)
     return QString::fromStdString(url);
 }
 
+QByteArray getFaviconData(const kiwix::Book& b)
+{
+    QByteArray qdata;
+    try {
+        // Try to obtain favicons only from local books (otherwise
+        // kiwix::Book::Illustration::getData() attempts to download the image
+        // on its own, whereas we want that operation to be performed
+        // asynchronously by ThumbnailDownloader).
+        if ( b.isPathValid() ) {
+            const auto illustration = b.getIllustration(48);
+            const std::string data = illustration->getData();
+
+            qdata = QByteArray::fromRawData(data.data(), data.size());
+            qdata.detach(); // deep copy
+        }
+    } catch ( ... ) {
+        return QByteArray();
+    }
+
+    return qdata;
+}
+
 QVariant getBookAttribute(const kiwix::Book& b, const QString& a)
 {
     if ( a == "id" )          return QString::fromStdString(b.getId());
@@ -304,6 +326,7 @@ QVariant getBookAttribute(const kiwix::Book& b, const QString& a)
     if ( a == "url" )         return QString::fromStdString(b.getUrl());
     if ( a == "name" )        return QString::fromStdString(b.getName());
     if ( a == "downloadId" )  return QString::fromStdString(b.getDownloadId());
+    if ( a == "favicon")      return getFaviconData(b);
     if ( a == "faviconUrl")   return getFaviconUrl(b);
     if ( a == "size" )        return QString::number(b.getSize());
     if ( a == "tags" )        return getBookTags(b);

src/contentmanagermodel.cpp

@@ -113,16 +113,8 @@ void ContentManagerModel::setBooksData(const BookInfoList& data)
 
 QByteArray ContentManagerModel::getThumbnail(const BookInfo& bookItem) const
 {
-    QString id = bookItem["id"].toString();
+    QByteArray bookIcon = bookItem["favicon"].toByteArray();
-    QByteArray bookIcon;
+    if ( bookIcon.isNull() ) {
-    try {
-        auto book = KiwixApp::instance()->getLibrary()->getBookById(id);
-        std::string favicon;
-        auto item = book.getIllustration(48);
-        favicon = item->getData();
-        bookIcon = QByteArray::fromRawData(reinterpret_cast<const char*>(favicon.data()), favicon.size());
-        bookIcon.detach(); // deep copy
-    } catch (...) {
         const auto faviconUrl = bookItem["faviconUrl"].toString();
         if (m_iconMap.contains(faviconUrl)) {
             bookIcon = m_iconMap[faviconUrl];
@@ -174,16 +166,11 @@ void ContentManagerModel::refreshIcons()
         return;
     td.clearQueue();
     for (auto i = 0; i < rowCount() && i < m_data.size(); i++) {
-        auto bookItem = m_data[i];
+        const auto& bookItem = m_data[i];
-        auto id = bookItem["id"].toString();
+        if ( bookItem["favicon"].toByteArray().isNull() ) {
-        const auto faviconUrl = bookItem["faviconUrl"].toString();
+            const auto faviconUrl = bookItem["faviconUrl"].toString();
-        auto app = KiwixApp::instance();
-        try {
-            auto book = app->getLibrary()->getBookById(id);
-            auto item = book.getIllustration(48);
-        } catch (...) {
             if (faviconUrl != "" && !m_iconMap.contains(faviconUrl)) {
-                td.addDownload(faviconUrl, id);
+                td.addDownload(faviconUrl, bookItem["id"].toString());
             }
         }
     }