Proposal: context-menu item for appending PV names to the clipboard

[abrahamwolk]

At the last codeathon, there was a discussion regarding #2998.

Since there seemed to be no consensus about adding the full functionality implemented in that ticket to Phoebus, I am now implementing this functionality as a site-specific plugin instead.

However, during the discussion at the codeathon, interest was expressed in adding just the context-menu entry that appends the PV names to the clipboard (without the descriptions/display names). Using this feature, it would be possible to "collect" a number of PV names from different OPIs, by copying the first PV name, and then appending further PV names to the clipboard using the "append PV name to clipboard"-functionality.

The proposal is to add such a "append PV name to clipboard"-menu item to the context menu.

[abrahamwolk]

I have created a PR with an implementation of this proposal here: #3103