Menus for single stepping

These are the menus for Single Stepping.  I have changed the menu layout
from how it was in 1.4 because I think it used to be confusing.  I
haven't added translations or actual single stepping functionality.

![image](https://cloud.githubusercontent.com/assets/7967463/3447980/0fe8f3a6-0154-11e4-9c91-1c5725e5369a.png)

Here is the .swf file if you would like to test it: https://www.dropbox.com/s/49ennf5lx7kwsce/Scratch.swf

If you know how to add the functionality to change the speed and to highlight the blocks that are running, that would be great! If not, I am still working!

This is my first time contributing to anything like this, so if I have done something wrong, please let me know.