* Moved several machine lifecycle callbacks to the notifier/subscriber
model. The old callback registration model is still available for
them for now, but prints a deprecation warning.
* Added pre-save/post-load notifications.
* Use a single allocated timer rather than one anonymous timer per
waiter. Waiters no longer prevent saved states from being loaded.
* Clean up outstanding waiters on stop or state load rather than just
leaking them.
* Started documenting parts of the emulator interface object that should
be relatively stable.
-imagedev/avivideo.cpp: Fixed an object leak on unload. Also changed
some other media image devices to use smart pointers.
* Also added a couple of missing command-line options, and added a local
table of contents to the (rather long) command line options page.
-Added a separate CI target for localisation updates that produces an
artefact, and removed the message catalogs from the trigger paths for
the Linux CI build.
