Downloader Re-work

This is a mix of both the hammond-downloader re-work and the way the gtk+ client handles downloads.

Proposed Mockups:

Design Tasks

• Create the mockup

Development Tasks

Switch from the custom downloader to a downloader crate

Refactor a bunch of stuff and tests to the new crate

Create the headerbar icon and Popover

Show headerbar icon only when there are active downloads

• Create a download manager that will "Own" and manage the download threads.
• Modify EpisodeWidget according to the mockuup

QA Tasks

• Assert downloading stuff works
• Assert download manager functions properly

Assert the download button is only shown when needed