Add Graphics#maximize_window

Works even if the window is not resizable by user.
Requested by matias1lol.

doc/extension_doc.md

@@ -1,3 +1,4 @@
+
 # mkxp extenions for Dancing Dragon Games / Symphony of War
 
 ## Graphics.resize_window(width, height, recenter)
@@ -8,6 +9,10 @@ returns: nil
 
 Resizes the game window to width x height. If `recenter` is **true**, also center the window on the current screen.
 
+## Graphics.maximize_window()
+
+Maximizes the game window, ignoring the `winResizable` config option.
+
 ## Bitmap.write_to_png(filename)
 filename: String  
 returns: self