Add preferences for screensaver age rating

[nielsvanvelzen]

This change adds age rating preferences for the screensaver. It depends on jellyfin/jellyfin#11367.

This is not the exact implementation that I wanted, ideally the maximum age rating preference would show the parental ratings from the server. There are two problems with it though:

• There is a breaking API change in 10.9 and the SDK we're currently using breaks because of it
  • Not a huge problem as 0.17 is most likely going to require 10.9, still annoying now that we're using the older SDK version
• The responses are not localized, some names are hardcoded (unrated/approved/banned)
  • And even if they used the server language, the client might want a different language. We'd need to add Accept-Language support to both the API and SDK for this to work properly. Which might be something to look at for 10.10, but definitely not 10.9

Adding the mapping via the server is something we can always add later though.

Changes

• Add preference for screensaver "require age rating"

  • Defaults to on (changed behavior since 0.16)
  • When turned on, only items with an age rating set will show
  • When turned off, all items will show

• Add preference for screensaver "maximum age rating"

  • Defaults to 13 (same behavior as 0.16)
  • Can also be set to a few different values, notable are 0 and -1.
  • -1 means "no maximum", parameter will be omitted in requests

Screenshots

Issues

Fixes #3320