add action cascadeActiveApp; add cascadeActiveAppWindowsOnScreen

[rudifa]

Changes:

• enum WindowAction: add case cascadeActiveApp and update functions that handle the cases
• class MultiWindowManager: add cascadeActiveAppWindowsOnScreen() and a call from execute()
• TerminalCommands.md: add section ## Add additional tiling and cascading commands

@rxhanson @dirondin

Please review the proposed changes and test them.

I tested the debug build on my Intel Mac (2017), and it works to my satisfaction.

Let me know if I missed anything. Kudos to both of you for the clean and understandable code, easy to extend.

@rudifa

[rudifa]

I also built an archive (with my build settings in the local version of .pbxproj).

It opens and runs successfully with my authorization.

[rxhanson]

Looks good. Thanks for contributing!

[rudifa]

Thank you for accepting! Cheers @rudifa