mainwindow: Disable page switcher while generation is running

This prevents 'spam'. User may have scrolled 10 pages forward, while
we are still generating old pages. Then the user wonders why
they arrive so late.

So disable switching pages while the generation is still running

It is unlikely that a user will have to quickly go through search
results like that.

gui/mainwindow.cpp

@@ -33,7 +33,11 @@ MainWindow::MainWindow(QWidget *parent, QString socketPath) : QMainWindow(parent
 	connect(&ipcPreviewClient, &IPCPreviewClient::previewReceived, this, &MainWindow::previewReceived,
 			Qt::QueuedConnection);
 	connect(&ipcPreviewClient, &IPCPreviewClient::finished, this,
-			[&] { this->ui->previewProcessBar->setValue(this->ui->previewProcessBar->maximum()); });
+			[&]
+			{
+				this->ui->previewProcessBar->setValue(this->ui->previewProcessBar->maximum());
+				this->ui->spinPreviewPage->setEnabled(true);
+			});
 	connect(&ipcPreviewClient, &IPCPreviewClient::error, this,
 			[this](QString msg)
 			{
@@ -471,6 +475,7 @@ void MainWindow::makePreviews(int page)
 	ui->previewProcessBar->setValue(0);
 	ui->previewProcessBar->setVisible(this->previewableSearchResults.size() > 0);
 	++this->currentPreviewGeneration;
+	this->ui->spinPreviewPage->setEnabled(false);
 	emit startIpcPreviews(renderConfig, targets);
 }