Fix crash when removing channels

Sometimes the application could crash when deleting a channel because it
would still be used when the method returned.

This commit makes the channel be deleted only after the stack returns to
the event loop.

Fixes LMMS#1679

include/FxMixer.h

@@ -25,6 +25,8 @@
 #ifndef FX_MIXER_H
 #define FX_MIXER_H
 
+#include <QObject>
+
 #include "Model.h"
 #include "Mixer.h"
 #include "EffectChain.h"
@@ -35,7 +37,7 @@
 class FxRoute;
 typedef QVector<FxRoute *> FxRouteVector;
 
-class FxChannel : public ThreadableJob
+class FxChannel : public ThreadableJob, public QObject
 {
 	public:
 		FxChannel( int idx, Model * _parent );

src/core/FxMixer.cpp

@@ -321,7 +321,7 @@ void FxMixer::deleteChannel( int index )
 	}
 
 	// actually delete the channel
-	delete m_fxChannels[index];
+	m_fxChannels[index]->deleteLater();
 	m_fxChannels.remove(index);
 
 	for( int i = index; i < m_fxChannels.size(); ++i )