Add a shorthand for setting the exit code using SceneTree::quit()

This reduces the amount of code required to exit a process with a
non-zero exit code. This pattern is also found in most other
programming languages.

doc/classes/SceneTree.xml

@@ -185,8 +185,10 @@
 		<method name="quit">
 			<return type="void">
 			</return>
+			<argument index="0" name="exit_code" type="int" default="-1">
+			</argument>
 			<description>
-				Quits the application.
+				Quits the application. A process [code]exit_code[/code] can optionally be passed as an argument. If this argument is [code]0[/code] or greater, it will override the [member OS.exit_code] defined before quitting the application.
 			</description>
 		</method>
 		<method name="reload_current_scene">