XKit Preferences: Improve error UI

[marcustyphoon]

This improves the UI shown in the XKit preference pane when there is an error accessing a script's data. Specifically, it fixes the title being "undefined" in certain circumstances, removes a backdrop element that made the text basically unreadable, and adds a button to force update extensions to fix the problem.

This is generally hard to encounter in practice (for this screenshot I ran browser.storage.local.remove('extension_old_blue') using Chrome's console's ability to run arbitrary code in the content script sandbox, which is very convenient). But it's an edge case that will come up if in a future version we execute scripts directly out of the extension package and don't save them to storage, but a user downgrades back to the current or a prior version... or, naturally, if you are a developer and are switching back and forth between branches that do and do not do this.

This should be copied into the 7.9.2 branch as well (note: copy the whole xkit_preferences.js and xkit_preferences.css files rather than cherry picking).

[marcustyphoon]

"Force mode" updates only extensions with new versions if they don't start with xkit_. Updating a specific installed extension from the extension panel does always work, though.

– #2194 (comment)

Kind of relevant here.