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/core/host_interface.h

@@ -50,10 +50,11 @@ public:
   /// Shuts down the emulator frontend.
   virtual void Shutdown();
 
-  bool BootSystem(const SystemBootParameters& parameters);
+  virtual bool BootSystem(const SystemBootParameters& parameters);
+  virtual void PowerOffSystem();
+
   void PauseSystem(bool paused);
   void ResetSystem();
-  void PowerOffSystem();
   void DestroySystem();
 
   /// Loads state from the specified filename.
@@ -158,6 +159,9 @@ protected:
   /// Sets the base path for the user directory. Can be overridden by platform/frontend/command line.
   virtual void SetUserDirectory();
 
+  /// Sets the user directory to the program directory, i.e. "portable mode".
+  void SetUserDirectoryToProgramDirectory();
+
   /// Performs the initial load of settings. Should call CheckSettings() and m_settings.Load().
   virtual void LoadSettings() = 0;
 
@@ -229,14 +233,14 @@ protected:
   Settings m_settings;
   std::string m_user_directory;
 
+  std::deque<OSDMessage> m_osd_messages;
+  std::mutex m_osd_messages_lock;
+
   bool m_paused = false;
   bool m_speed_limiter_temp_disabled = false;
   bool m_speed_limiter_enabled = false;
   bool m_timer_resolution_increased = false;
 
-  std::deque<OSDMessage> m_osd_messages;
-  std::mutex m_osd_messages_lock;
-
 private:
   void InitializeUserDirectory();
   void CreateAudioStream();