Queue async tasks
This MR grew organically and provides a few different things.
- Adds a queue for async tasks, this increases smoothness during startup and doesn't hammer the system with a ton of IO early on. Thisshould also fix the reported problems with too many open files at once.
- Move
DefaultIcon
out ofArtCache
and clean it up a bit. - Load the default icons in the views right away to make for a cleaner experience.