Make classes final

[FelixJacobi]

fix #121.

The main BigBlueButton class is also marked as @final. I decided against to also make it final as this is breaking DX too much in my opinion. We would need an Interface to allow that, or you are not able to mock the class anymore. This should be postponed to version 5.1+. For now, it is a reminder, that you should not extend the class in your code.

• This PR points to 5.0 branch. 5.0 will be developed with 4.3 in parallel, like explained in Policy for BC breaks and feature changes #122.
• 5.0 will have the same features as 4.3 minus Deprecations and plus BC Breaks.
• When 4.3 was branched and released, 5.0 branch will be merged back to master. Until this is done, every merge to master requires an "upmerge" to the 5.0 branch.

[FelixJacobi]

Merged as it got the required peer approvals already.