frontend: data lake: allow deleting variables in the monitor

[ES-Alexander]

Current behaviour

Data lake variables cannot be deleted, even if they were created by an Input Widget or Action that no longer exists.

Expected or desired behaviour

• [x] We should be able to delete them directly, using the data lake variables monitor
  • Ideally this would be limited to those that are created by user input rather than those used by Cockpit's internals and/or auto-generated from receiving MAVLink messages, but "allow deleting everything" could be an ok starting point as long as there's a way to restore required values if something gets broken (hopefully just by refreshing the page)
  • Initial deletion support (from the table) was implemented in #1706
• [ ] Per this comment, it would be nice if we could detect whether a variable is being used anywhere else, and delete it automatically when the last reference to it is removed
  • This is a form of garbage collection, and could potentially be achieved using reference counting (e.g. store a count of references to a variable with the variable, increment that whenever it's referred to, and decrement that when a reference is removed/deleted, then delete the variable when the count decrements to 0)

Prerequisites

• [x] I have checked to make sure that a similar request has not already been filed or fixed.