UI: Set focus back to label after source rename

Fixes obsproject/obs-studio#3465

UI/source-tree.cpp

@@ -354,6 +354,7 @@ void SourceTreeItem::ExitEditMode(bool save)
 	editor = nullptr;
 	setFocusPolicy(Qt::NoFocus);
 	boxLayout->insertWidget(index, label);
+	label->setFocus();
 
 	/* ----------------------------------------- */
 	/* check for empty string                    */