Update Gradle and introduce Kotlin to NewPipe

[mauriciocolli]

Update Gradle and introduce Kotlin

I think it's time we introduce Kotlin to NewPipe as I consider it very nice to use. The main reasons/opinions as to why I think it'd be good to the project:

• I want to learn more about it, so why not have more contact with it
• Want to learn something new
• Relative ease to migrate and interact with Java code
• Enjoyable coding overall

If you think this is not a good addition for NewPipe, please post your thoughts below.

[Koitharu]

I think, we do not need Kotlin in this project, aren`t?

[TobiGr]

IMO Kotlin simplifies code a lot: easy null checks, less unnecessary brackets, etc.
@theScrabi We had the discussion somewhere before. I'd approve Kotlin support. What's you opinion?

Apart from that, #2309 is written in Kotlin and I'd like to get this merged asap.

[Stypox]

I am in favour of the introduction of Kotlin, since it is faster to program in and the transition from Java is smooth