Better queue management

[XenorPLxx]

When uses chooses the option to keep the library in sync constantly, and the plugin can't synchronize for whatever reason, the queue of games to synchronize is lost, which means some games might have their data not up to date in Sharenite until those games are synchronized again or a full library synchronization is done.

The goal of this task is to make sure we're doing everything we can on client side to always keep games in sync.

Implementing better queue management should resolve any issues with queue being lost due to server availability and other unforeseen, but temporary, errors. It should also address issues like #17.

CTA:

• queue should be retried in growing time intervals to make sure all new changes are uploaded.
• when a game is added to deleted queue, it should be removed from updated queue
• when option to keep in sync is not checked, update queue should be cleared out and turned off

Questions:

• what are the reasonable time intervals, so user is not spammed with notifications?
• should authentication error also be included as standard auto retry? If not:
  • should queue be still maintained, but sync paused (and re-triggered after user authenticates)?
  • or should queue be cleared out as it is if unrecoverable error happens now?

Prerequisites:

• not necessary, but good to have Change authentication check method #9
• should be done in one swoop, maybe? Refactor code #5

BTW: Sharenite does not synchronize changes instantly, instead it keeps a queue and synchronize only if there are no changes for a couple of seconds, to prevent multiple API calls during library updates, hence the 'queue' exists.