[Icon Item] Also clear image icon when using Plasma Svg

When changing an IconItem from using a local image (e.g. PNG file) to using a Plasma themed icon,
the implicit size wasn't updated since the old image was still loaded in m_imageIcon.
Clearing it when clearing the other icon stores fixes this.

BUG: 405298

Differential Revision: https://phabricator.kde.org/D19674

src/declarativeimports/core/iconitem.cpp

@@ -187,6 +187,7 @@ void IconItem::setSource(const QVariant &source)
             //success?
             if (m_usesPlasmaTheme && m_svgIcon->isValid() && m_svgIcon->hasElement(sourceString)) {
                 m_icon = QIcon();
+                m_imageIcon = QImage();
                 m_svgIconName = sourceString;
                 //ok, svg not available from the plasma theme
             } else {