Fix PlasmaCore.IconItem initialization when using QPixmap on it

I was getting the m_pixmapIcon cleared some times, instead of the
m_iconPixmap. This made it forget about my image.
m_iconPixmap should be renamed to something else.

Reviewed and done in collaboration with David Edmindson

src/declarativeimports/core/iconitem.cpp

@@ -314,7 +314,7 @@ void IconItem::loadPixmap()
     //final pixmap to paint
     QPixmap result;
     if (size <= 0) {
-        m_pixmapIcon = QPixmap();
+        m_iconPixmap = QPixmap();
         m_animation->stop();
         update();
         return;