feat: add taskbar progress

[shatyuka]

Description

Add a download progress indicator to the taskbar.

Details:

• Show total progress of multiple tasks, excluding tasks of unknown size
• Task of unknown size will show an indeterminate progress bar on Windows, or zero progress on other platforms
• Lower priority than update progress

Related Issues

Resolves #633, resolves #918, resolves #1249

Checklist:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you linted your code locally prior to submission?
• Have you successfully ran app with your changes locally?