Here's a quick list of actions that can be reversed while building apps.
Deleting Steps/Widgets
When you delete a step or widget, a small "Undo" button will appear in the middle of the screen at the top of the App Editor below the Menu Bar.
It will remain on the screen for 10 seconds after you have deleted that widget. You can only undo the last "delete" action.
App Editor Events
Using Control+Z (or CMD+Z on Mac), you can undo the last event in each of these following categories:
Moving a widget
Resizing a widget
Rotating a widget