Comment 9 for bug 1125442

@Marco
I am thinking about a fix for this:
* Add a setting to change the current behavior (e.g. Remember viewport focus)
* Before changing viewport save the currently focused window
* When coming back to a viewport instead of focusing the default window do:
i) if the new setting is enabled try to restore the focus (if the window is still there)
ii) else fallback to the default behavior (focusDefaultWindow)

I want to know your opinion on this approach before starting the implementation.

Do you think it makes sense to implement this in core (src/) or is it rather specific to wall plugin and should be implemented there (along with the setting in wall plugin settings)?