Libvlc v4 support

[oaubert]

The coming libvlc v4 has a number of breaking changes that will requires multiple changes in the code generator and templates.

Breaking Changes:

• libvlc_event_manager_t has been removed. Objects now take versioned libvlc_*_cbs callback structs at creation. Use the migration guide to map old event handlers.
• libvlc_time_t is now measured in microseconds across the public API. Every function that used milliseconds now accepts/returns microseconds via libvlc_time_t.
• libvlc_media_t parsing and thumbnailing APIs have been replaced by libvlc_parser_t.
• Anonymous unions in public structs have been replaced with named unions for better compatibility with auto-generated bindings.
• Several APIs have been renamed or removed (see the migration guide).

New features:

• libvlc_parser_t: unified parsing and thumbnailing API with its own thread pool, concurrent requests, and allows re-queueing already parsed media.
• Versioned callback structs: append-only design provides forward ABI compatibility, allowing newer libvlc.so versions to run existing binaries unchanged.
• libvlc_media_player_cbs: complete replacement for MediaPlayer events, includes new functionality like on_media_attachments_added callback delivering a libvlc_picture_list_t of attachments, and on_media_stopping now exposes a libvlc_stopping_reason_t (fixes #26819).
• Media list player rewritten to listen directly to player callbacks.
• libvlc_downloader API: asynchronously downloads media as a bitstream with progress reporting, enabling local storage, caching, etc. It is introduced in !7848, which is on top of this MR and will be merged afterwards.