Improve Firefox on-update cleanup

[marcustyphoon]

Concept

I wasn't thinking of modifying runScript at all; I was thinking of modifying init to detect if XKit has already run, then calling destroyScript for each enabled script if it has (and calling $('style.xkit').remove()) before continuing with the usual .forEach(runScript).

To achieve the desired effect from this, each feature's clean() method will need to be slightly adjusted to not rely on references to elements in memory, and tested for if it errors out before its main() is run (I'm looking at you, Old Sidebar in XKit 7). Beyond that, it's just a matter of making the utility modules clean themselves up like the sidebar/meatballs utils or repetition-prevent themselves like the remixicon util.

Originally posted by @AprilSylph in #451 (comment)