UI: Fix potential race condition

The signal mutex can be locked within other signal mutexes, causing a
potential hard cross-lock when releasing certain sources.  Instead,
defer the Disconnect() call to the UI thread.

This fixes a potential freeze on exit.

UI/source-tree.cpp

@@ -129,6 +129,12 @@ void SourceTreeItem::DisconnectSignals()
 	removeSignal.Disconnect();
 }
 
+void SourceTreeItem::Clear()
+{
+	DisconnectSignals();
+	sceneitem = nullptr;
+}
+
 void SourceTreeItem::ReconnectSignals()
 {
 	if (!sceneitem)
@@ -150,10 +156,8 @@ void SourceTreeItem::ReconnectSignals()
 					Q_ARG(OBSSceneItem, curItem));
 			curItem = nullptr;
 		}
-		if (!curItem) {
-			this_->DisconnectSignals();
-			this_->sceneitem = nullptr;
-		}
+		if (!curItem)
+			QMetaObject::invokeMethod(this_, "Clear");
 	};
 
 	auto itemVisible = [] (void *data, calldata_t *cd)

UI/source-tree.hpp

@@ -72,6 +72,8 @@ private:
 	virtual void paintEvent(QPaintEvent* event) override;
 
 private slots:
+	void Clear();
+
 	void EnterEditMode();
 	void ExitEditMode(bool save);