Use a custom layout for upgrader

[Leilei332]

Closes #9221

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[Leilei332]

The problem of using a custom layout is that it will cause conflict if user's tiddlywiki contains the $:/layout tiddler. Though we can move $:/layout to the plugin, the layout still changes after the user imports the tiddlers, and the "Download upgraded wiki" screen won't show up, which breaks consistency.

[Jermolene]

The problem of using a custom layout is that it will cause conflict if user's tiddlywiki contains the $:/layout tiddler. Though we can move $:/layout to the plugin, the layout still changes after the user imports the tiddlers, and the "Download upgraded wiki" screen won't show up, which breaks consistency.

It is an interesting problem. Perhaps it would require some custom JS in the upgrade plugin to accomplish this.

The issue is of course broader, in that we have problems if the wiki being upgraded overwrites any shadow tiddlers required for the operation of the upgrader itself. It would be interesting to explore whether we could build an upgrader that somehow isolated the tiddlers being upgraded. It would be straightforward to store the incoming tiddlers in a temporary plugin that has a custom plugin-type. We would need a means to modify the tiddlers within a plugin. For saving, we would need a way to reuse the existing templates while using a custom store area that extracts the payload tiddlers as subtiddlers from the temporary plugin.

If it were feasible, I think this would be a very worthwhile improvement for v5.4.0. The current behaviour of seeing your customisations (mis-)applied to the upgrader itself is jarring and unexpected for users.