Fix issue #570 Make it harder to accidentally modify a shadow tiddler

Original issue #570 

If a user accidentally clicks edit on the control panel tiddler, and then clicks the tick to revert to view mode then they will override the shadow control panel tiddler with a modified version. As a result, even after an upgrade, you'd still be looking at the old control panel wikitext.

It's too easy for this to happen accidentally. Some things we can do to alleviate the problem:

* if the tick is clicked without the tiddler having changed in any way, perhaps we should not save the tiddler. This would mean that editing and clicking the tick would not update the modified time of a tiddler

* we could prompt the user for confirmation before overwriting a shadow tiddler for the first time

* we could show an annotation banner over shadow tiddlers explaining what they are (as in Classic TiddlyWiki)