Update poster and art and move to a mixin

[JonnyWong16]

Description

• Separate out Poster and Art objects. These are just the exact same BasePosterArt object with a different name so it's more user friendly.
• Move poster and art methods to mixins (based on the idea from Move split/merge and unmatch/match to a mixin #639). This way the methods are only available to the objects that have posters and artwork.
• Added some tests for posters and art.

Breaking change:

• Poster.selected is casted to abool instead of being a str

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable