We should not be able to enqueue another worker when a worker is waiting
or is operating on a RareGame. To avoid that, set the RareGame's state as
soon as the worker is instatiated and refuse to add another worker if one
is already atteched to the RareGame.

Uploading or downloading saves takes some time occasionally. Add
the operation as a queue worker to be visible in the bottom status bar

Because syncing is a network operation and verify/move are disk IO
operation, a second threadpool and queue were added to contain separate
them and queue the workers independently. One threadpool handles disk io
tasks and the other networking tasks with a maximum of 2 active threads
for each of them.