Allow disabling QgsNetworkAccessManager timeout functionality

by setting the timout to 0

This can be interesting to overcome the limitations of the timeout
functionality by replacing it with QNetworkAccessManager::setTransferTimeout.
E.g. to have more than 6 downloads running in parallel.

python/core/auto_generated/qgsnetworkaccessmanager.sip.in

@@ -267,6 +267,7 @@ Returns the network timeout length, in milliseconds.
     static void setTimeout( int time );
 %Docstring
 Sets the maximum timeout ``time`` for network requests, in milliseconds.
+If set to 0, no timeout is set.
 
 .. seealso:: :py:func:`timeout`
 

src/core/qgsnetworkaccessmanager.cpp

@@ -276,15 +276,18 @@ QNetworkReply *QgsNetworkAccessManager::createRequest( QNetworkAccessManager::Op
   // The timer will call abortRequest slot to abort the connection if needed.
   // The timer is stopped by the finished signal and is restarted on downloadProgress and
   // uploadProgress.
-  QTimer *timer = new QTimer( reply );
-  timer->setObjectName( QStringLiteral( "timeoutTimer" ) );
-  connect( timer, &QTimer::timeout, this, &QgsNetworkAccessManager::abortRequest );
-  timer->setSingleShot( true );
-  timer->start( timeout() );
-
-  connect( reply, &QNetworkReply::downloadProgress, timer, [timer] { timer->start(); } );
-  connect( reply, &QNetworkReply::uploadProgress, timer, [timer] { timer->start(); } );
-  connect( reply, &QNetworkReply::finished, timer, &QTimer::stop );
+  if ( timeout() )
+  {
+    QTimer *timer = new QTimer( reply );
+    timer->setObjectName( QStringLiteral( "timeoutTimer" ) );
+    connect( timer, &QTimer::timeout, this, &QgsNetworkAccessManager::abortRequest );
+    timer->setSingleShot( true );
+    timer->start( timeout() );
+
+    connect( reply, &QNetworkReply::downloadProgress, timer, [timer] { timer->start(); } );
+    connect( reply, &QNetworkReply::uploadProgress, timer, [timer] { timer->start(); } );
+    connect( reply, &QNetworkReply::finished, timer, &QTimer::stop );
+  }
   QgsDebugMsgLevel( QStringLiteral( "Created [reply:%1]" ).arg( reinterpret_cast< qint64 >( reply ), 0, 16 ), 3 );
 
   return reply;

src/core/qgsnetworkaccessmanager.h

@@ -424,6 +424,7 @@ class CORE_EXPORT QgsNetworkAccessManager : public QNetworkAccessManager
 
     /**
      * Sets the maximum timeout \a time for network requests, in milliseconds.
+     * If set to 0, no timeout is set.
      *
      * \see timeout()
      * \since QGIS 3.6