Frontends: Add shared command line interface

Also provides batch mode and automatic fullscreen switching.

  -help: Displays this information and exits.
  -version: Displays version information and exits.
  -batch: Enables batch mode (exits after powering off)
  -fastboot: Force fast boot for provided filename
  -slowboot: Force slow boot for provided filename
  -resume: Load resume save state. If a boot filename is provided,
    that game's resume state will be loaded, otherwise the most
    recent resume save state will be loaded.
  -state <index>: Loads specified save state by index. If a boot
    filename is provided, a per-game state will be loaded, otherwise
    a global state will be loaded.
  -statefile <filename>: Loads state from the specified filename.
    No boot filename is required with this option.
  -fullscreen: Enters fullscreen mode immediately after starting.
  -nofullscreen: Prevents fullscreen mode from triggering if enabled.
  -portable: Forces "portable mode", data in same directory.
  --: Signals that no more arguments will follow and the remaining
    parameters make up the filename. Use when the filename contains
    spaces or starts with a dash.

src/duckstation-qt/mainwindow.cpp

@@ -528,6 +528,7 @@ void MainWindow::connectSignals()
   connect(m_host_interface, &QtHostInterface::systemPerformanceCountersUpdated, this,
           &MainWindow::onSystemPerformanceCountersUpdated);
   connect(m_host_interface, &QtHostInterface::runningGameChanged, this, &MainWindow::onRunningGameChanged);
+  connect(m_host_interface, &QtHostInterface::exitRequested, this, &MainWindow::close);
 
   // These need to be queued connections to stop crashing due to menus opening/closing and switching focus.
   connect(m_game_list_widget, &GameListWidget::entrySelected, this, &MainWindow::onGameListEntrySelected,