feat: support custom shortcut for existing actions

Resolve https://github.com/BLumia/pineapple-pictures/issues/128

Actually also https://github.com/BLumia/pineapple-pictures/issues/72
but not sure why that one is now a 404 page.

app/mainwindow.cpp

@@ -151,6 +151,7 @@ MainWindow::MainWindow(QWidget *parent)
 
     QTimer::singleShot(0, this, [this](){
         m_am->setupShortcuts();
+        Settings::instance()->applyUserShortcuts(this);
     });
 
     // allow some mouse events can go through these widgets for resizing window.