Add left and right arrow support to window classes

[fmatthew5876]

RPG_RT supports down an up arrows on windows to show that the window can scroll.

For our debug scene, we now have multiple pages of commands but when you access the debug scene it's not at all obvious that you can do more things by using the left and right keys.

Add support for left and right arrows, similar to the up and down ones, and try using them on the debug scene command window.

This is a good project for beginners who want to get into EasyRPG coding.