Add game_handle_event()

This allows users to choose how certain Allegro events are handled.
Add register_event_source()

This allows the user to add extra event sources that aren't used directly
by Taiga.
Separate Allegro library uses onto different lines.

The old way, the target_link_libraries line was far too long.
Make TaigaState::init() initialize Allegro primitives.
Change actor list from std::list to std::vector.

The actor list only holds pointers to actors, not actual actors.
Because these are relatively small, removals in the middle of
the container don't take much time to do. In this case,
std::list doesn't actually perform much faster than std::vector
in practice anyway.