* 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.
* util/options.cpp: Added option types for single and multiple paths.
* util/options.cpp: Substitute environment variables in values from defaults and INI files.
* ui/dirmenu.cpp: Removed hard-coded list of multi-path options.
* plugins: Don't substitute environment variables in path options.
Added emulated time recording as well as wall clock time.
Fixed recording time for multiple software items per system. An
incorrect constraint on the database table meant that time was only
being recorded for a single software item per system.
Detect the "empty" driver so the time spent at the selection menu isn't
recorded (you'd get multiple entries for this due to the way options
leak when returning to the system selection menu).
Included schema migration code to update existing timer plugin
databases. Also replaced some unnecessary floating point code with
integer maths, added log messages, and made the plugin unload unload its
database access code during emulation.
Changed other plugins' use of paths with trailing slashes as this causes
stat to fail on Windows.
