Step 10 of the gui/app.py + gui/preview.py structural refactor. Pure
copy: the QObject signal hub that BooruApp uses to marshal async
worker results back to the GUI thread moves to its own module. All
14 signals are preserved verbatim. app.py grows another shim line
so internal `AsyncSignals()` references in BooruApp keep working.
Shim removed in commit 14.
