[Headless] Add --headless switch (no rendering, no audio).

Also remove now unused "--no-window" option, and relative OS getter and
setter.

core/os/os.h

@@ -53,7 +53,6 @@ class OS {
 	bool _verbose_stdout = false;
 	bool _debug_stdout = false;
 	String _local_clipboard;
-	bool _no_window = false;
 	int _exit_code = EXIT_FAILURE; // unexpected exit is marked as failure
 	int _orientation;
 	bool _allow_hidpi = false;
@@ -269,9 +268,6 @@ public:
 
 	virtual Error move_to_trash(const String &p_path) { return FAILED; }
 
-	virtual void set_no_window_mode(bool p_enable);
-	virtual bool is_no_window_mode_enabled() const;
-
 	virtual void debug_break();
 
 	virtual int get_exit_code() const;