gui: Disable search box as long as previews are being generated

Otherwise "spamming" queries can cause high load and
many outstanding may arrive only to be discarded anyway
for not being part of the recent query.

gui/mainwindow.cpp

@@ -215,6 +215,7 @@ void MainWindow::connectSignals()
 				this->ui->previewProcessBar->setValue(this->ui->previewProcessBar->maximum());
 				this->ui->spinPreviewPage->setEnabled(true);
 				this->ui->comboPreviewFiles->setEnabled(true);
+				ui->txtSearch->setEnabled(true);
 			});
 	connect(&ipcPreviewClient, &IPCPreviewClient::error, this,
 			[this](QString msg)
@@ -955,6 +956,7 @@ void MainWindow::makePreviews(int page)
 	++this->currentPreviewGeneration;
 	this->ui->spinPreviewPage->setEnabled(false);
 	this->ui->comboPreviewFiles->setEnabled(false);
+	this->ui->txtSearch->setEnabled(false);
 	emit startIpcPreviews(renderConfig, targets);
 }