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/system.h

@@ -35,7 +35,9 @@ struct SystemBootParameters
   ~SystemBootParameters();
 
   std::string filename;
+  std::string state_filename;
   std::optional<bool> override_fast_boot;
+  std::optional<bool> override_fullscreen;
 };
 
 class System