Enhancement: Informational popup when New XKit is updated in Firefox

[marcustyphoon]

In Firefox, automatic extension updates cause the extension to be reloaded, in-place, in every currently open Tumblr tab. This can cause all sorts of bugs (such as duplicating the New XKit control button) unless the extension is very carefully designed.

This seems particularly important to handle in a Tumblr extension because many users seem to keep the same Tumblr tabs for weeks or months(?), per the large number of issues I see reported in Tumblr posts that indicate that this is being done and are fixed with a simple refresh.

We could make a popup appear when this occurs rather than silently having potentially erroneous behavior, perhaps with an optional refresh-this-page button.

We declined to do this in XKit Rewritten (AprilSylph/XKit-Rewritten#451) in favor of attempting to fix most possible erroneous behavior that results from these updates. This was pretty complicated, so it doesn't make much sense as a strategy for this codebase.