Update steam-taskbar-progress to v1.2.1

[luthor112]

Taskbar Download progress

• Replace backedn library with rewrite
• Do task on download completion
• Pause/unpause all downloads
• Fix loading, update libs, update ReadMe, bump version

Task Checklist

Developer

• I am the original author or an authorized maintainer of this plugin.
• I have complied with all license requirements for the libraries used, including providing appropriate notices where necessary.
• My plugin is fully open source and does not depend on any external paid services, except for widely trusted and well-known platforms. Additionally, neither I nor anyone associated with me profits from any such services.

Plugin Functionality

• I have tested the plugin on both the Stable and Beta SteamOS update channels.
• Should this really be "SteamOS", or just "Steam"?
• If just Steam, isn't Steam Beta unsupported by Millennium?

Backend Configuration

• Yes: I use a standard Millennium python backend in my plugin.
• No: I use custom binaries that or rely on other FOSS projects that aren't written directly using Millennium's python backend.

Community Contribution

• I have tested and left feedback on two other plugin pull requests.
• I have added links to those feedback comments in this PR.

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Testing Instructions

• Verified by a third party on Steam Client Stable.
• Verified by a third party on Steam Client Beta.