The main reason for this is the compositing of both local and online covers with the same logic.
It was a problem raised in #146 with some covers getting stretched.
Changes:
- Renamed and simplified managers methods
- Created a generic `cover manager`
- Added more retryable errors to `steam api manager`
- Removed `local cover manager` and `online cover manager`
- Reduced dependency on `PIL`
Changed run_after, retryable_on and continue_on to be type Container.
We don't need them to be sets.
The performance gain of sets over small tuples is nonexistant
for in checks and the syntax is more verbose.
