Fix FIFO usage

`m_propriorityQueue` and `m_queue` are used as FIFO queues, so we need
to use `takeFirst()` instead of `takeLast()` when removing elements
from the queue.

While at it, add a comment so that we will remember these are FIFO
queues.

src/kitemviews/private/kdirectorycontentscounter.cpp

@@ -104,7 +104,7 @@ void KDirectoryContentsCounter::slotResult(const QString& path, int count, long
     }
 
     if (!m_priorityQueue.isEmpty()) {
-        startWorker(m_priorityQueue.takeLast());
+        startWorker(m_priorityQueue.takeFirst());
     } else if (!m_queue.isEmpty()) {
         startWorker(m_queue.takeFirst());
     }

src/kitemviews/private/kdirectorycontentscounter.h

@@ -72,6 +72,7 @@ private:
 private:
     KFileItemModel* m_model;
 
+    // Used as FIFO queues.
     QLinkedList<QString> m_priorityQueue;
     QLinkedList<QString> m_queue;