feat: change task container in ModDownloadDialog to a QHash

Previously, we used a unique_ptr to a ModDownloadTask to keep track of
the selected mod to download when we accepted the dialog.

In order to allow multiple mods to be selected at once for download,
this has been changed to a QHash where the key is the mods name (since
it doesn't seem right to allow for multiple versions of the same mod to
be downloaded at once), and the value is a pointer to the corresponding
ModDownloadTask.

launcher/ModDownloadTask.h

@@ -10,6 +10,7 @@ class ModDownloadTask : public Task {
     Q_OBJECT
 public:
     explicit ModDownloadTask(const QUrl sourceUrl, const QString filename, const std::shared_ptr<ModFolderModel> mods);
+    const QString& getFilename() const { return filename; }
 
 public slots:
     bool abort() override;