Add option to control qBittorrent process memory priority

This is to avoid Windows swapping out other application data from
OS cache and put in torrent data which hinders other program
responsiveness.

The default value "Below normal" is choosen because casual users have
other higher priority apps (such as browser) running and they don't
expect OS to swap out its data. Dedicated seeders most probably will not
have other app running on their system and would expect qbt has
priority over other background services.

The option only has effect on Windows >= 8.

src/base/bittorrent/session.h

@@ -132,6 +132,18 @@ namespace BitTorrent
             AntiLeech = 2
         };
         Q_ENUM_NS(SeedChokingAlgorithm)
+
+#if defined(Q_OS_WIN)
+        enum class OSMemoryPriority : int
+        {
+            Normal = 0,
+            BelowNormal = 1,
+            Medium = 2,
+            Low = 3,
+            VeryLow = 4
+        };
+        Q_ENUM_NS(OSMemoryPriority)
+#endif
     }
     using namespace SessionSettingsEnums;
 
@@ -387,6 +399,10 @@ namespace BitTorrent
         void setTrackerFilteringEnabled(bool enabled);
         QStringList bannedIPs() const;
         void setBannedIPs(const QStringList &newList);
+#if defined(Q_OS_WIN)
+        OSMemoryPriority getOSMemoryPriority() const;
+        void setOSMemoryPriority(OSMemoryPriority priority);
+#endif
 
         void startUpTorrents();
         TorrentHandle *findTorrent(const InfoHash &hash) const;
@@ -530,6 +546,9 @@ namespace BitTorrent
         void populateAdditionalTrackers();
         void enableIPFilter();
         void disableIPFilter();
+#if defined(Q_OS_WIN)
+        void applyOSMemoryPriority() const;
+#endif
 
         bool addTorrent_impl(CreateTorrentParams params, const MagnetUri &magnetUri,
                              TorrentInfo torrentInfo = TorrentInfo(),
@@ -661,6 +680,9 @@ namespace BitTorrent
         CachedSettingValue<bool> m_isDisableAutoTMMWhenCategorySavePathChanged;
         CachedSettingValue<bool> m_isTrackerEnabled;
         CachedSettingValue<QStringList> m_bannedIPs;
+#if defined(Q_OS_WIN)
+        CachedSettingValue<OSMemoryPriority> m_OSMemoryPriority;
+#endif
 
         // Order is important. This needs to be declared after its CachedSettingsValue
         // counterpart, because it uses it for initialization in the constructor