Add 'Export Data to CSV' option to plot context menu

[lars-nagel]

Summary

• Adds right-click context menu option "Export Data to CSV" to plots
• Exports only data within the visible/zoomed time range
• Supports multiple curves merged by timestamp

Motivation

Addresses the feature request for per-plot CSV export respecting zoom level (see issue #1218).

Currently users must use the global CSV Publisher to export data all loaded data. This adds a quick way to export just the visible portion of a specific plot.

[facontidavide]

Before you work more on this, please take into account that most probably I will not merge it in.

I am planning a complete refactoring of the CSV export to be much better than it currently is, therefore this change may not fit in my plan

[facontidavide]

Thanks a lot, but as I said, I will do this differenlty.

I appreciate your contribution and I promise I will give you something even better than this in the next few months