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/qthostinterface.cpp

@@ -39,6 +39,11 @@ QtHostInterface::~QtHostInterface()
   Assert(!m_display_widget);
 }
 
+const char* QtHostInterface::GetFrontendName() const
+{
+  return "DuckStation Qt Frontend";
+}
+
 bool QtHostInterface::Initialize()
 {
   createThread();
@@ -104,6 +109,12 @@ bool QtHostInterface::ConfirmMessage(const char* message)
   return result;
 }
 
+bool QtHostInterface::parseCommandLineParameters(int argc, char* argv[],
+                                                 std::unique_ptr<SystemBootParameters>* out_boot_params)
+{
+  return CommonHostInterface::ParseCommandLineParameters(argc, argv, out_boot_params);
+}
+
 QVariant QtHostInterface::getSettingValue(const QString& name, const QVariant& default_value)
 {
   std::lock_guard<std::recursive_mutex> guard(m_qsettings_mutex);
@@ -320,6 +331,11 @@ bool QtHostInterface::SetFullscreen(bool enabled)
   return true;
 }
 
+void QtHostInterface::RequestExit()
+{
+  emit exitRequested();
+}
+
 std::optional<CommonHostInterface::HostKeyCode> QtHostInterface::GetHostKeyCode(const std::string_view key_code) const
 {
   const std::optional<int> code =