Extended interface to control custom windows

[numeredev]

DESCRIPTION

What does your feature request improve on? Please describe. The custom window feature lacks some interaction features like maximize or change of a selection. This makes it impossible to implement e.g. a search feature in a custom window.

Describe the solution you'd like Provide an extended interface to custom windows, especially the following command sequences:

## Get or set a selection
window ID -set item=ID selection=ID
window ID -get item=ID selection=ID

## Set the keyboard focus
window ID -set [item=ID] focus

## Maximize or restore a custom window
window ID -set maximize/restore

## Modify the dropdown lists for dropdown and combobox
window ID -set item=ID label={LABELS}

Additional context Add any other context or screenshots about the feature request here.

(Do not write below this line)

---

DEVS' SECTION

ANALYSIS

To be done like usual window commands.

IMPLEMENTATION STEPS

(see also our Wiki for implementation guidelines)

• Implement the necessary changes in a new branch created here on GitHub
• Test your implementation

DOCUMENTATION STEPS

(see also our Wiki for further information)

• Update the changes log
• Add comments to your implementation
• Add Doxygen documentation comments
• Create or update the documentation articles (*.NHLP and *.NDB files, if needed)
• Update the language strings (*.NLNG files, if needed)

PULL REQUEST

• Create a pull request for your changes
• Fill out the template
• Assign @numere-org/maintainers as reviewers