feat(meta,core,app,android,ios): cast and trailer

AniList's fragment now asks for the first ten characters, main roles
first, with their Japanese voice actor, and for the trailer's site and
id. Both are stored in the existing details JSON as TitleDetails.cast and
.trailer with serde defaults, so no migration is needed and details
stored before them still read. The details REVISION goes to 2, so a
stored match is refetched by its id on the next metadata run and the
cast fills in without anyone matching the title again.

The title page shows a Cast row of round portraits with the character's
name over the actor's. Clicking a tile opens the Person browse page for
that actor, which now also admits voice actors, and search counts them
under People and studios. A Trailer button opens the YouTube or
Dailymotion page in the browser; the app itself makes no request to
either, and an unknown site or an id with odd characters gives no link.

docs/USER_GUIDE.md

@@ -114,6 +114,12 @@ top.
   franchise, and shows one title from each other franchise. It needs posters and descriptions.
 - The genre chips under the description open a page with every title of that genre in your
   library. So do the **Studio** and **Director** chips. Back, or **Esc**, returns to the title.
+- **Cast** lists the main characters and who voices them in Japanese. Click one to see every title
+  in your library with that voice actor. Search finds voice actors too.
+- **Trailer** opens the title's trailer in your web browser. The app does not play it or load it
+  itself.
+- Titles matched before this version get their cast and trailer by themselves the next time the
+  app looks up posters and descriptions.
 - With posters and descriptions on, a show's logo takes the place of its name at the top of the
   page and on the library's banner, and each episode says how long it runs. A film says how long
   it runs in the line under its name.