fix: 尝试修复 ISSUE #I68I8W 安装结束时随机崩溃问题

暂时未知，目前怀疑安装结束后，DownloadItem 触发 finished() 信号，在详情页槽函数中执行 disconnect 操作时导致崩溃

Log: 去除 disconnect 操作；修改可能造成问题的控件指针获取方式；修复多线程执行下载安装操作时跨线程相关警告

* 该提交可能存在严重问题，建议多进行测试；如果存在问题，及时 revert 该提交

src/widgets/downloadlistwidget.h

@@ -20,7 +20,7 @@ class DownloadListWidget : public DBlurEffectWidget
     Q_OBJECT
 
 public:
-    void addItem(QString name, QString fileName, QString pkgName, const QPixmap icon, QString downloadurl);
+    DownloadItem *addItem(QString name, QString fileName, QString pkgName, const QPixmap icon, QString downloadurl);
     int nowDownload = 0;
     int allDownload = 0;
     QList<DownloadItem *> getDIList();
@@ -35,7 +35,7 @@ private:
     QStringList dlist;
     QList<QUrl> urList;
     QList<DownloadItem *> downloaditemlist;
-    DownloadController *downloadController;
+    DownloadController *downloadController = nullptr;
     int nowdownload = 0;
     QString theSpeed;
     QTimer download_speed;