ContentManagerModel accesses ContentManager directly

Indirect access from ContentManagerModel to ContentManager (via
KiwixApp) contained a race condition that could lead to a crash.

Here is the call chain:

- In KiwixApp::init() the KiwixApp::mp_manager data member is initialized
  as follows:

    mp_manager = new ContentManager(&m_library, mp_downloader);

- ContentManager's constructor creates the model and the view:

    mp_view = new ContentManagerView();
    managerModel = new ContentManagerModel(this);
    updateModel();
    auto treeView = mp_view->getView();
    treeView->setModel(managerModel);
    treeView->show();

- This starts a cascade of asynchronous events that will eventually
  result in ContentManagerModel::sort() being executed. The latter has
  to call ContentManager::sortBy() and if it does so indirectly via
  `KiwixApp::getContentManager()` before the constructor of
  ContentManager has completed (and KiwixApp::mp_manager has been
  assigned to) a crash is imminent.

This commit eliminates that issue.

src/contentmanagermodel.cpp

@@ -7,8 +7,9 @@
 #include "kiwixapp.h"
 #include <kiwix/tools.h>
 
-ContentManagerModel::ContentManagerModel(QObject *parent)
+ContentManagerModel::ContentManagerModel(ContentManager *contentMgr)
-    : QAbstractItemModel(parent)
+    : QAbstractItemModel(contentMgr)
+    , m_contentMgr(*contentMgr)
 {
     connect(&td, &ThumbnailDownloader::oneThumbnailDownloaded, this, &ContentManagerModel::updateImage);
 }
@@ -225,7 +226,7 @@ void ContentManagerModel::sort(int column, Qt::SortOrder order)
         default:
             sortBy = "unsorted";
     }
-    KiwixApp::instance()->getContentManager()->setSortBy(sortBy, order == Qt::AscendingOrder);
+    m_contentMgr.setSortBy(sortBy, order == Qt::AscendingOrder);
 }
 
 RowNode* ContentManagerModel::getRowNode(size_t row)

src/contentmanagermodel.h

@@ -11,6 +11,7 @@
 #include "rownode.h"
 #include <memory>
 
+class ContentManager;
 class RowNode;
 class Node;
 class DescriptionNode;
@@ -58,7 +59,7 @@ public: // types
 
 
 public: // functions
-    explicit ContentManagerModel(QObject *parent = nullptr);
+    explicit ContentManagerModel(ContentManager* contentMgr);
     ~ContentManagerModel();
 
     QVariant data(const QModelIndex &index, int role) const override;
@@ -89,6 +90,7 @@ private: // functions
     RowNode* getRowNode(size_t row);
 
 private: // data
+    ContentManager& m_contentMgr;
     std::shared_ptr<RowNode> rootNode;
     mutable ThumbnailDownloader td;
     QMap<QString, size_t> bookIdToRowMap;