Resize the notebook windows

[szabgab]

The editor pages within the notebook can be now dragged around but there are few arrangements that seem to make particular sense.

While I am on a specific window I'd like to say "split horizontally"
or "split vertically" that would minimize (or hide somehow)
all the other editors. Split the current document so there are two copies of it just as the Window/Split? window does now and arranges them one next to the other or one below the other.