added fallback icon for thumbnail sync context menu

The sync icon was missing when using the Breeze Icon pack.
Now a sync icon and a refresh icon is used for this thumbnail sync action.

part/part.cpp

@@ -2997,8 +2997,10 @@ void Part::showMenu(const Okular::Page *page, const QPoint point, const QString
     QAction *fitPageWidth = nullptr;
     if (page) {
         popup->addAction(new OKMenuTitle(popup, i18n("Page %1", page->number() + 1)));
-        if (m_thumbnailList->isVisible() && !Okular::Settings::syncThumbnailsViewport())
-            popup->addAction(QIcon::fromTheme(QStringLiteral("action-synchronize")), i18n("Sync Thumbnail with Page"), m_thumbnailList.data(), &ThumbnailList::syncThumbnail);
+        if (m_thumbnailList->isVisible() && !Okular::Settings::syncThumbnailsViewport()) {
+            const QIcon &syncIcon = QIcon::fromTheme(QStringLiteral("emblem-synchronizing"), QIcon::fromTheme(QStringLiteral("view-refresh")));
+            popup->addAction(syncIcon, i18n("Sync Thumbnail with Page"), m_thumbnailList.data(), &ThumbnailList::syncThumbnail);
+        }
         if ((!currentPage && m_document->bookmarkManager()->isBookmarked(page->number())) || (currentPage && m_document->bookmarkManager()->isBookmarked(m_document->viewport())))
             removeBookmark = popup->addAction(QIcon::fromTheme(QStringLiteral("bookmark-remove"), QIcon::fromTheme(QStringLiteral("edit-delete-bookmark"))), i18n("Remove Bookmark"));
         else