Unsolo mixer channels on delete (#3982)

If a mixer channel is soloed when it's deleted the other channels are left in the
state their in which is, for the most part, muted. Solve this by clearing mixer
channels on delete.

src/gui/FxMixerView.cpp

@@ -382,6 +382,10 @@ void FxMixerView::deleteChannel(int index)
 	// remember selected line
 	int selLine = m_currentFxLine->channelIndex();
 
+	// in case the deleted channel is soloed or the remaining
+	// channels will be left in a muted state
+	Engine::fxMixer()->clearChannel(index);
+
 	// delete the real channel
 	Engine::fxMixer()->deleteChannel(index);