Cleanup DebugManager

[sluicebox]

I had trouble reading DebugManager to figure out recent regressions so I cleaned it up.

• Renamed methods and members to be consistent and clearer
• Updated doxygen comments to be in sync with method behaviors
• Global debug channels are now automatically added by DebugManager. The code was going to a lot of effort to try to achieve that but still not always succeeding. Now it's explicit and simpler. Callers don't have to always call two methods to add channels.
• Removed unnecessary calls to remove debug channels from Engine destructors. As of PR BASE: add getDebugChannels in metaEngineDetection #3012, Engines are no longer responsible for adding their channels, so this code should have been removed from the destructors. It's redundant; runGame() already handles this.

[ysj1173886760]

Thank you, that reverting part is correct. I didn't managed to get the total sight of this, so i made some mistakes.
Thank you for amending and optimizing those codes. I've learnt a lot though that. Much Appreciated.

[sluicebox]

Thanks for reviewing this!

The confusion in this class predates your changes -- channels vs flags, adding vs registering vs setting, etc. Now that all that code is in one place, those existing inconsistencies became clearer. I also like that you've got the channels managed outside of the Engine constructors and destructors.

Now that the expert has taken a look, I'm comfortable merging. =)