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
- Cleanup obsolete code here and there

doc/base/classes.xml

@@ -14960,13 +14960,6 @@
 	<description>
 	</description>
 	<methods>
-		<method name="get_custom_level" qualifiers="const">
-			<return type="String">
-			</return>
-			<description>
-				Returns the value of the commandline argument "-level".
-			</description>
-		</method>
 		<method name="get_frames_drawn">
 			<return type="int">
 			</return>