fix(player): add descendants option in contentchildren

[tomstolarczuk]

Description

This option is required in new Angular version, otherwise it doesn't know how to collect nested children in ng-content.

Checklist

Please, check that you have been followed next steps:

• I've read the (contributing)[/~https://github.com/videogular/ngx-videogular/blob/master/CONTRIBUTING.md] guidelines
• Code compiles correctly (run npm start)
• Created tests (if necessary)
• All tests passing (run npm test)
• Extended the README / documentation (if necessary)

[IxquitilisSaid]

LGTM. Thanks for the contribution!