Add notification settings, which allows muting decky/plugin toast notifications

[JohnnyCrazy]

Please tick as appropriate:

• I have tested this code on a steam deck or on a PC
• My changes generate no new errors/warnings
• This is a bugfix/hotfix
• This is a new feature

If you're wanting to update a translation or add a new one, please use the weblate page: https://weblate.werwolv.net/projects/decky/

Description

This fixes issue: #473

[ttang4299]

Hey @JohnnyCrazy , I don't know if there's room to implement a further change, but if you know how to toggle notifications for successful and/or unsuccessful update checks (or at least add a section where it can show when a successful update check was last performed), but if you know if this could be implemented, it would be very cool if you could add this whenever you have the time to

Reference: #425

[Kalidibus]

@JohnnyCrazy - Thank you so much for this! The notifications were a huge immersion breaker when in game.