Popup: Enable user selection on version number

[marcustyphoon]

Concept

Welcome to: the most niche feature request ever!*

Right now you can't copy the XKit Rewritten version number out of the extension popup to paste it into the issue form because the whole popup has user-select: none.

This would be convenient to override for that element (or just to disable entirely; sometimes I try to copy the text of a preference checkbox in order to tell a user which checkbox does the thing they want to do.)

*disclaimer: statement is probably false; I'm quite sure I have made significantly niche-er feature requests.