Support fallback when selecting theme icons

Fallback icon theme are not supported everywhere. Hence we mimic
signature of QIcon::fromTheme().

src/gui/guiiconprovider.cpp

@@ -57,10 +57,17 @@ GuiIconProvider *GuiIconProvider::instance()
 }
 
 QIcon GuiIconProvider::getIcon(const QString &iconId)
+{
+    return getIcon(iconId, iconId);
+}
+
+QIcon GuiIconProvider::getIcon(const QString &iconId, const QString &fallback)
 {
 #if (defined(Q_OS_UNIX) && !defined(Q_OS_MAC))
     if (m_useSystemTheme) {
-        QIcon icon = QIcon::fromTheme(iconId, QIcon(IconProvider::getIconPath(iconId)));
+        QIcon icon = QIcon::fromTheme(iconId);
+        if (icon.name() != iconId)
+            icon = QIcon::fromTheme(fallback, QIcon(IconProvider::getIconPath(iconId)));
         icon = generateDifferentSizes(icon);
         return icon;
     }

src/gui/guiiconprovider.h

@@ -44,6 +44,7 @@ public:
     static GuiIconProvider *instance();
 
     QIcon getIcon(const QString &iconId);
+    QIcon getIcon(const QString &iconId, const QString &fallback);
     QIcon getFlagIcon(const QString &countryIsoCode);
     QString getIconPath(const QString &iconId);