Fixes for new two-dash long command line arguments

- Fixes some single-dash leftovers that were missed in the previous commit
- Reorder the help output for clarity, and document missing options
- Drop obsolete options: --noop, --pack, --editor-scene, --level, --import, --import-script, --no-quit
- Improve error message on malformed arguments and do not display help on error
- Always use long form of arguments when starting a new Godot process from C++, for clarity and easy grepping
- Cleanup obsolete code here and there

core/engine.h

@@ -39,7 +39,6 @@ class Engine {
 
 	friend class Main;
 
-	String _custom_level;
 	uint64_t frames_drawn;
 	uint32_t _frame_delay;
 	uint64_t _frame_ticks;
@@ -70,8 +69,6 @@ public:
 
 	virtual float get_frames_per_second() const { return _fps; }
 
-	String get_custom_level() const { return _custom_level; }
-
 	uint64_t get_frames_drawn();
 
 	uint64_t get_fixed_frames() const { return _fixed_frames; }