Add new native media player synchronisation functions

[caprica]

https://code.videolan.org/videolan/vlc/-/commit/e648e23872d4321c8e6a2b3157d8022336cc40fa

[caprica]

Not really sure how useful these will be for a Java app, or whether vlcj should be using them internally.

In theory it's a way to synchronise multiple operations without the native player going off on its own and doing something asynchronously, it could be useful just not sure of the use-cases yet.