Fixed handling of arrow keys on focused layer combo box

Using the arrow keys with the combo box selected only changed the layer
shown in the combo box, but failed to actually change the selected
layer.

Closes #1973

src/tiled/treeviewcombobox.cpp

@@ -101,25 +101,32 @@ void TreeViewComboBox::wheelEvent(QWheelEvent *e)
 
 void TreeViewComboBox::keyPressEvent(QKeyEvent *e)
 {
+    QModelIndex index;
+
     if (e->key() == Qt::Key_Up || e->key() == Qt::Key_PageUp) {
-        setCurrentModelIndex(indexAbove(m_view->currentIndex()));
+        index = indexAbove(m_view->currentIndex());
     } else if (e->key() == Qt::Key_Down || e->key() == Qt::Key_PageDown) {
-        setCurrentModelIndex(indexBelow(m_view->currentIndex()));
+        index = indexBelow(m_view->currentIndex());
     } else if (e->key() == Qt::Key_Home) {
-        QModelIndex index = m_view->model()->index(0, 0);
+        index = m_view->model()->index(0, 0);
         if (index.isValid() && !(model()->flags(index) & Qt::ItemIsSelectable))
             index = indexBelow(index);
-        setCurrentModelIndex(index);
     } else if (e->key() == Qt::Key_End) {
-        QModelIndex index = lastIndex(m_view->rootIndex());
+        index = lastIndex(m_view->rootIndex());
         if (index.isValid() && !(model()->flags(index) & Qt::ItemIsSelectable))
             index = indexAbove(index);
-        setCurrentModelIndex(index);
     } else {
         QComboBox::keyPressEvent(e);
         return;
     }
 
+    if (index.isValid()) {
+        setCurrentModelIndex(index);
+
+        // for compatibility we emit activated with a useless row parameter
+        emit activated(index.row());
+    }
+
     e->accept();
 }