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/node/project/footage/footage.cpp

@@ -22,6 +22,7 @@
 #include "footage.h"
 
 #include <QApplication>
+#include <QDebug>
 #include <QDir>
 #include <QStandardPaths>
 
@@ -229,12 +230,24 @@ void Footage::SetSourceStartTime(const rational &time, const QString &source)
 	has_source_start_time_ = true;
 }
 
+void Footage::set_proxy_enabled(bool enabled)
+{
+	if (proxy_enabled_ != enabled) {
+		qDebug() << "Footage::set_proxy_enabled:" << filename() << enabled;
+		proxy_enabled_ = enabled;
+		emit ProxySettingsChanged();
+	}
+}
+
 void Footage::SetProxy(const QString &path,
 					   ProxyManager::ProxyState state,
 					   int video_stream_index,
 					   int preset_version,
 					   bool enabled)
 {
+	qDebug() << "Footage::SetProxy:" << filename() << "enabled=" << enabled
+			 << "state=" << ProxyManager::ProxyStateToString(state)
+			 << "path=" << path;
 	proxy_path_ = path;
 	proxy_state_ = state;
 	proxy_video_stream_index_ = video_stream_index;

app/node/project/footage/footage.h

@@ -178,13 +178,7 @@ public:
 		return proxy_enabled_;
 	}
 
-	void set_proxy_enabled(bool enabled)
-	{
-		if (proxy_enabled_ != enabled) {
-			proxy_enabled_ = enabled;
-			emit ProxySettingsChanged();
-		}
-	}
+	void set_proxy_enabled(bool enabled);
 
 	const QString &proxy_path() const
 	{