Add ability to automatically upgrade core code from tiddlywiki.com

[JeremyRuston]

Following Saq's contribution of a proof of concept  here, add the ability for a TiddlyWiki document to automatically upgrade itself with the latest core code from tiddlywiki.com

[JeremyRuston]

The first part of this ticket is in changeset:4256.

It adds a user interface to Saq's code, and does the backing up, the downloading and saving of the core code, and the reloading of the page, but it doesn't yet implement the #upgrade paramifier.

[JeremyRuston]

The final part of this ticket is in changeset:4332

[FND]

This is looking good! If I understand this correctly though, there are a few issues:

• using alert() to confirm the upgrade might be a bit crude - a proper UI notification might be better here
• there is a link to CoreUpgrades on the community wiki - however, that page does not exist yet (as discussed before, there are also some issues with establishing that kind of "official" link between TiddlyWiki itself and the community resource at this point)
• it seems that an upgrade is performed regardless of whether a new core version is available - a conditional message displaying the current status would be highly desirable
• there is no check to prevent upgrading while viewing over HTTP

[FND]

About the issue of checking for a new version before upgrading, this could be achieved in a number of ways (as discussed in the  original thread):

• Cook generates a simple version info file, which is checked when opening the upgrade wizard
• instead of having Cook generate the version info file, the SVN repository's  version.js might be used.
• instead of using a version info file, the latest empty.html's version info is checked

For obvious reasons, the first option is the most desirable one.