Add Achievements Manager plugin

[ifantom]

Achievements Manager

Provide control over your achievements.
You can mark any achievement as "ignored" and it will be hidden from any places except new "Ignored achievements" tab on Achievements page where you can reconsider it.

Checklist:

Developer Checklist

• I am the original author or an authorized maintainer of this plugin.
• I have abided by the licenses of the libraries I am utilizing, including attaching license notices where appropriate.

Plugin Checklist

• I have verified that my plugin works properly on the Stable and Beta update channels of SteamOS.
• I have verified my plugin is unique or alternatively provides more/alternative functionality to a similar plugin already on the store.

Plugin Backend Checklist

• No: I am using a custom backend other than Python.
• No: I am using a tool or software from a 3rd party FOSS project that does not have it's dependencies statically linked.
• No: I am using a custom binary that has all of it's dependencies statically linked.

Testing

• Tested on SteamOS Stable/Beta Update Channel.

• Tested on SteamOS Preview Update Channel.