Make `LayoutCommand`s callables without prompting the user

[kl0tl]

This allows to call save_layout, restore_layout, remove_layout and new_window_from_saved_layout with a nickname argument to execute the command without asking the user for it. Useful to trigger those commands programmatically.

There is a bit of refactor involved to extract the common code between those commands into a LayoutCommand base class. It makes the derived commands much more lean but I could revert those changes if you find it’s too much.

[adzenith]

I need to test this at some point. It looks good from skimming it.