Implement clipboard paste workaround for Firefox

[david-a-wheeler]

• Firefox does not implement the clipboard readText() function (and hasn't for years). This implements a workaround so that the "paste" button works within metamath-lamp while running Firefox, that is, you can "copy to clipboard" and later "paste from clipboard". This workaround means "copy to clipboard" works completely normally, and that's what most users will care about. Note that "paste from clipboard" only copies data from within the application while running Firefox, because that's all we can do. That's still much better than not working at all. This works fine in Chrome, and if Firefox ever implements clipboard readText(), this code will quietly switch to using it.

[david-a-wheeler]

This fix is a very good workaround solution which definitely works and makes copy/paste much more convenient.

Thanks!

I left few small comments. Please take a look.

Thanks, I've incorporated them. I think it's ready to merge.

[david-a-wheeler]

Thanks for the merge!

It's up to you, but I think there is more than enough for a release. If undo is almost ready, then great, but there's no need to wait for a while.

[expln]

Thanks for the help with this new feature! I've just recently released it. Yes, undo is almost ready, it will be another release.