Set n-window extent: default and current value (non-GUI)

[hjoliver]

We're getting quite a few requests for this, and it would be easy to do (and of course, we do plan to do it ... this probably features in an existing issue somewhere, but I failed to find it).

Currently the default is fixed (n=1) and users can only change it via the GUI.

Users need to be able to:

• configure the default value
• change the current value via the CLI and TUI

The appropriate (or at least the maximum appropriate) n-window extent is kinda workflow-dependent, and possibly user-dependent (e.g. for users who typically only run small workflows), so I think it would make sense to set the default n-window extent via both a global and workflow config item.