Add an overlay prompt for enabling saving

[Jermolene]

There's a new prerelease of TiddlyFox 2.0.2 that incorporates a new overlay warning prompt:

Please give it a try and report back before I submit it to Mozilla. Unfortunately, using unsigned extensions is difficult these days. The easiest way is to use the Developer edition of Firefox, which will accept unsigned extensions:

https://www.mozilla.org/en-GB/firefox/developer/

The XPI is available here:

https://github.com/TiddlyWiki/TiddlyFox/raw/master/tiddlywiki.xpi

[pmario]

Hi, If I install and open FF dev edition on ubuntu 17.04 TiddlyFox icon looks like this:

So it's basically invisible. ... IMO we need a new icon.

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I do have several TWs, where I intentionally disable saving. ... So this overlay is way to aggressive for this special usecase.

[pmario]

I'd prefer a new Page-Toolbar icon that points out, that there is something wrong, or could be enabled.

[Jermolene]

So it's basically invisible. ... IMO we need a new icon.

There appears to be no way for an extension to choose a different icon based on whether the theme is light or dark. I think the best we can do is to add a subtle white border: