gui: Clear previews always on new search results

If a first search generates previews and the next one does
not, old entries would still have been visible.

So we just clear them once we get results.

gui/mainwindow.cpp

@@ -588,6 +588,8 @@ void MainWindow::lineEditReturnPressed()
 void MainWindow::handleSearchResults(const QVector<SearchResult> &results)
 {
 	this->previewableSearchResults.clear();
+	qDeleteAll(ui->scrollAreaWidgetContents->children());
+
 	ui->treeResultsList->clear();
 	ui->comboPreviewFiles->clear();
 	ui->comboPreviewFiles->addItem("All previews");