Add diagnostics for proxy toggle synchronization

Add temporary qDebug logging to Footage, ProjectCopier,
RenderWorkerPool, and TimelineWidget to trace why Use Proxy cannot be
 toggled and why proxy footage may still be used after disabling.

app/widget/timelinewidget/timelinewidget.cpp

@@ -1216,8 +1216,12 @@ void TimelineWidget::GenerateProxiesForSelectedClips()
 void TimelineWidget::SetSelectedClipsProxyEnabled(bool enabled)
 {
 	const QVector<Footage *> footage = GetSelectedProxyFootage(selected_blocks_);
+	qDebug() << "TimelineWidget::SetSelectedClipsProxyEnabled:" << enabled
+			 << "footage count=" << footage.size();
 	for (Footage *item : footage) {
 		if (item->proxy_path().isEmpty()) {
+			qDebug() << "  skipping item with empty proxy path"
+					 << item->filename();
 			continue;
 		}