An improvement to 6fb992690d
By making sure that all the popup state tiddlers have the prefix
$:/state/popup/ we can be more selective and just exclude popup state
tiddlers when we save. The end result is that once again other kinds of
state are persisted as expected (eg tabs, and sidebar visibility).
This might be a controversial change as it also means that it is no
longer possible to include `$:/state/…` tiddlers in a wiki to set up
the initial configuration.
