UI: Refresh source tree when an item source is removed

Previously, calling `obs_source_remove()` on an input would not fully
remove it if there was a scene item of the input on the current scene.

Now, when calling `obs_source_remove()` and there is a scene item of
the removed input on the current scene, the UI refreshes the scene
items, allowing the scene item to be released completely and the
input destroyed.

UI/source-tree.cpp

@@ -311,6 +311,7 @@ void SourceTreeItem::ReconnectSignals()
 			reinterpret_cast<SourceTreeItem *>(data);
 		this_->DisconnectSignals();
 		this_->sceneitem = nullptr;
+		QMetaObject::invokeMethod(this_->tree, "RefreshItems");
 	};
 
 	obs_source_t *source = obs_sceneitem_get_source(sceneitem);