Fix IconItem not loading non-theme icons with name

Commit 5184ac introduced a small regression that caused IconItem
to no longer load non-theme icons with name set, for instance icons
coming from sni-qt, causing sni-qt apps to not show in systray.

BUG: 359388
FIXED-IN: 5.20
REVIEW: 127091

src/declarativeimports/core/iconitem.cpp

@@ -148,6 +148,10 @@ void IconItem::setSource(const QVariant &source)
                 //fail, use QIcon
                 } else {
                     m_icon = QIcon::fromTheme(sourceString);
+                    if (m_icon.isNull()) {
+                        // fallback for non-theme icons
+                        m_icon = source.value<QIcon>();
+                    }
                     delete m_svgIcon;
                     m_svgIcon = 0;
                     m_imageIcon = QImage();