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/frontend-common/common_host_interface.h

@@ -33,19 +33,34 @@ public:
 
   using HotkeyInfoList = std::vector<HotkeyInfo>;
 
+  /// Returns the name of the frontend.
+  virtual const char* GetFrontendName() const = 0;
+
   virtual bool Initialize() override;
   virtual void Shutdown() override;
 
+  virtual bool BootSystem(const SystemBootParameters& parameters) override;
+  virtual void PowerOffSystem() override;
+
   /// Returns a list of all available hotkeys.
   ALWAYS_INLINE const HotkeyInfoList& GetHotkeyInfoList() const { return m_hotkeys; }
 
   /// Access to current controller interface.
   ALWAYS_INLINE ControllerInterface* GetControllerInterface() const { return m_controller_interface.get(); }
 
+  /// Returns true if running in batch mode, i.e. exit after emulation.
+  ALWAYS_INLINE bool InBatchMode() const { return m_batch_mode; }
+
+  /// Parses command line parameters for all frontends.
+  bool ParseCommandLineParameters(int argc, char* argv[], std::unique_ptr<SystemBootParameters>* out_boot_params);
+
 protected:
   CommonHostInterface();
   ~CommonHostInterface();
 
+  /// Request the frontend to exit.
+  virtual void RequestExit() = 0;
+
   virtual bool IsFullscreen() const;
   virtual bool SetFullscreen(bool enabled);
 
@@ -85,4 +100,7 @@ private:
 
   // input key maps
   std::map<HostKeyCode, InputButtonHandler> m_keyboard_input_handlers;
+
+  // running in batch mode? i.e. exit after stopping emulation
+  bool m_batch_mode = false;
 };