* Don't try to expand a screensaver message without any special tokens
* Don't needlessly nil the background when show_message is true
Honor the actual nil background setting, now that we've got one.
Also, only clear covers_fullscreen with no background *and* no widget.
* Support special tokens outside of ReaderUI. Because the undocumented
"Oops, I'll use the fallback otherwise" was extremely not
user-friendly.
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