FilterContainer.java:
=====================
This class is a container that keeps either content filters or sort filters organized.
Sort/content filters ({@link FilterItem}s) are organized within {@link FilterGroup}s.

FilterGroup.java:
=================
This class represents a filter category/group. For example 'Sort order'.

Its main purpose is to host a bunch of {@link FilterItem}s that belong to that
group. Eg. 'Relevance', 'Views', 'Rating'

FilterItem.java:
================
This class represents a single filter option.
*More in detail:*
For example youtube offers the filter group 'Sort order'. This group
consists of filter options like 'Relevance', 'Views', 'Rating' etc.
-> for each filter option a FilterItem has to be created.

BaseSearchFilters.java:
=======================
The base class for every service describing their {@link FilterItem}s,
{@link FilterGroup}s, the relation between content filters and sort filters.

…earch/filter/FilterGroup.java

Available content filters:
- All
- Videos
- Channels
- Playlists
Sepia
- SepiaSearch

Available sort filters:
- 'Sort by'
- stream kind
- sensitive
- 'Date'
- 'Duration'

Available content filters:
- All
- Tracks
- Users
- Playlists

Available sort filters:
- 'Sort by'
- Length
- license

Available content filters:
- All
- Conferences
- Events

Available content filters:
- All
- artists & labels
- albums
- tracks

    Available content filters:
    Youtube
    - all
    - videos
    - channels
    - playlists
    Youtube Music
    - Songs
    - Videos
    - Albums
    - Playlists
    - Artists

    Available sort filters:
    - 'Sort by'
    - Upload Date
    - Duration
    - Features

Without that code the search results will complete and the gui will not
spin forever.

… within the searchfilters

The class LibraryStringIds is a enum.
This enum's identify a message. The name of the enum constants
reflect the actual message. This message has to be transformed by the
library user into a meaningful string.

…onnull

…nnull

Annotate methods in classes that got some signatures changed due to the
searchfilters integration

DividerItem was inserted in the content filter framework in the
NewPipeExtractor to have a section title for YoutubeMusic. But as
UI releated stuff seems a bit out of place in the Extractor I came
up with injecting the DividerItem aka section title in the frontend
without having to change too much in the frontend.

…laylists search

'ALL' option not possible as Peertube uses different
endpoints for video/channels/playlist search

…ontent filter search

…able encountered IOException

The following Exception crashes NewPipe. Adding Serializable to FilterItem solves it:

java.lang.RuntimeException: Parcelable encountered IOException writing serializable object
(name = org.schabi.newpipe.extractor.linkhandler.ListLinkHandler)
...
Caused by: java.io.NotSerializableException: org.schabi.newpipe.extractor.search.filter.FilterItem