支持图标跟随主题，改进详情页，加入更好的多线程下载

新的下载会重试一个线程上的错误，一个线程崩溃次数过多会转移到队列里等待重新安排，其他的暂时没写

inc/spkdownload.h

@@ -0,0 +1,102 @@
+
+#pragma once
+
+#include "spkstore.h"
+
+/**
+ * @note SpkDownloadMgr does NOT do download scheduling and other things; it's only a multithreaded
+ *       downloader; it manages the threads that are downloading stuff from the Internet.
+ */
+
+class SpkDownloadMgr : public QObject
+{
+    Q_OBJECT
+  public:
+    SpkDownloadMgr(QObject *parent = nullptr);
+    ~SpkDownloadMgr();
+
+    static constexpr int MaximumThreadRetryCount = 3;
+
+    enum TaskResult
+    {
+      Success = 0,
+      FailCannotCreateFile, ///< Failed because destination file cannot be created
+      FailNoVaibleServer, ///< Failed because no server provides file size or download stalled on
+                          ///< all of them
+      Fail
+    };
+
+    /**
+     * @brief DownloadWorker is not a real worker but more of a worker-like structure. It holds
+     *        information about one download thread, such as offset, total bytes and received bytes.
+     *        Workers' scheduling is still done by SpkDownloadMgr synchronously, avoiding
+     *        unnecessary race conditions and data safety problems.
+     *        DownloadWorker is also used in mFailureRetryQueue to indicate the blocks that needed
+     *        to be retried on other servers.
+     */
+    struct DownloadWorker
+    {
+      QNetworkReply *Reply; ///< Reply from the network
+      qint64 BeginOffset; ///< Where should a worker start downloading
+      qint64 BytesNeeded; ///< How many bytes a worker should fetch in total
+      qint64 BytesRecvd; ///< How many bytes a worker has received till now
+    };
+
+    struct RemoteFileInfo
+    {
+      qint64 Size = -1;
+      bool SupportPartialDownload = false; ///< Whether this file can be downloaded multithreaded
+      QByteArray Md5;
+    };
+
+    void SetNewServers(QList<QString> servers);
+
+    /**
+     * @note This function uses BLOCKING IO!
+     */
+    static RemoteFileInfo GetRemoteFileInfo(QUrl url);
+
+  private:
+    QList<QString> mServers; ///< Multithreaded download
+
+    QList<DownloadWorker> mScheduledWorkers;
+
+    // If one scheduled task fails a few times in a row, we must give it up on that server and put
+    // its responsible block onto this queue so we can try downloading the block from other servers
+    QQueue<DownloadWorker> mFailureRetryQueue;
+
+    QFile mDestFile;
+    QString mDestFolder, mCurrentRemotePath;
+    RemoteFileInfo mCurrentRemoteFileInfo;
+
+    int mCurrentDownloadId;
+    int mActiveWorkerCount;
+
+  public slots:
+    void SetDestinationFolder(QString path);
+
+    /**
+     * @brief StartNewDownload try to start new download task.
+     * @param path File path. Domain name excluded. No leading slashes.
+     * @param downloadId Emitted with progress, finish and error so the UI know whose status it is.
+     * @return true for success and false for failure.
+     */
+    bool StartNewDownload(QString path, int downloadId);
+    bool PauseCurrentDownload();
+    bool CancelCurrentDownload();
+
+  private slots:
+    void WorkerFinish();
+    void WorkerDownloadProgress(); ///< Be connected to ***QNetworkReply::readyRead***
+
+  private:
+    void LinkReplyWithMe(QNetworkReply*);
+    void TryScheduleFailureRetries();
+    void TryScheduleFailureRetries(int i); ///< Try schedule on a specific task slot.
+
+  signals:
+    void DownloadProgressed(qint64 bytes, qint64 total);
+    void DownloadStopped(TaskResult status, int id);
+
+
+};