[nostalgia] Update nested namespaces for C++17

src/nostalgia/studio/mainwindow.cpp

@@ -29,8 +29,7 @@
 
 #include "mainwindow.hpp"
 
-namespace nostalgia {
-namespace studio {
+namespace nostalgia::studio {
 
 const QString MainWindow::StateFilePath = "studio_state.json";
 
@@ -375,4 +374,3 @@ void MainWindow::refreshProjectExplorer(QString path) {
 }
 
 }
-}