Notify on extension import failure

[marcustyphoon]

This code is to easily test #636, which I am still trying to reproduce on any other computer.

User-facing changes

• Displays a (persistent) notification if any of the XKit scripts take longer than three seconds to import on the initial extension load. When a script successfully imports, its notification is removed, so only scripts that are currently not active are persistently displayed.

Of course, this only allows this behavior to be tested when the extension is loaded unpacked; if the Chrome web store version did not ever cause this, it would be mostly irrelevant. I have had the Chrome web store version do the same thing, though.

Technical explanation

Issues this closes