Ability to choose which libraries are used for background images

[ponchohoncho]

Is your feature request related to a problem? Please describe.
There is a blurred out background image that is pulled from a random Plex title that a logged in user has access to. If a user has access to sensitive/nsfw content on that server this means that the Synclounge homepage can show sensitive media, even if the user isn't engaging with it. Ask me how I found out...

Describe the solution you'd like
Either as a server owner or a user, I'd like to be able to choose which libraries screenshots can be pulled from e.g. Movies & TV Shows only