qt: add fetchMore ListCache

this allows to request more data from the cache using the standart growth policy
rather than requiring the caller to `refer` and item after what is currently
loaded. This function ensure that the cache tries to load at least the current
count + chunk size

modules/gui/qt/util/listcache.hpp

@@ -235,6 +235,11 @@ public:
      */
     void refer(size_t index);
 
+    /**
+     * Request to fetch more data
+     */
+    void fetchMore();
+
     /*
      * reload
      */

modules/gui/qt/util/listcache.hxx

@@ -374,6 +374,26 @@ void ListCache<T>::refer(size_t index)
     }
 }
 
+template<typename T>
+void  ListCache<T>::fetchMore()
+{
+    const CacheData* cache = m_cachedData.get();
+    if (!cache)
+        cache = m_oldData.get();
+    if (!cache)
+        return; //nothing loaded yet
+
+    if (cache->loadedCount >= cache->queryCount)
+        return;
+
+    if (m_maxReferedIndex >= cache->loadedCount + m_chunkSize)
+        return;
+
+    m_maxReferedIndex = std::min(cache->loadedCount + m_chunkSize, cache->queryCount);
+    if (!m_appendTask)
+        asyncFetchMore();
+}
+
 template<typename T>
 void ListCache<T>::invalidate()
 {