Add new methods to edit object fields

[JonnyWong16]

Description

Adds a new EditFieldMixin to edit Plex object fields (title, summary, content rating, etc.).

• New base method EditFieldMixin.editField()

Each of the following field mixin subclasses EditFieldMixin.

• Common fields: ContentRatingMixin, OriginallyAvailableMixin, OriginalTitleMixin, SortTitleMixin, StudioMixin, SummaryMixin, TaglineMixin, TitleMixin
• Unique for tracks: TrackArtistMixin, TrackNumberMixin, TrackDiscNumberMixin
• Unique for photos: PhotoCapturedTimeMixin

Adds new batch editing mode to save all edits in a single API call.

• New methods PlexPartialObject.batchEdits() and PlexPartialObject.saveEdits().

Examples:

# Single edits
Movie.editTitle('A New Title', locked=True)
Show.editSummary('A summary of the show.')
Episode.editContentRating('TV-14')
Artist.editStudio('New Record Label')
Track.editTrackArtist('New Artist')

# Chaining multiple edits with reloading (individual API call for each edit)
Movie.editTitle('A New Title').editSummary('A new summary').editTagline('A new tagline').reload()

# Batch editing multiple fields and tags in a single API call
Movie.batchEdits()
Movie.editTitle('A New Title').editSummary('A new summary').editTagline('A new tagline') \
    .addCollection('New Collection').removeGenre('Action').addLabel('Favorite')
Movie.saveEdits()

Closes #354

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• 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