Refactor code based on review

[hbeni]

James Turner was so glad to provide some comments on the code:

https://sourceforge.net/p/flightgear/mailman/flightgear-devel/thread/3C2C3D58-6F58-4949-879E-261978FD24F0%40flightgear.org/#msg37165208

• Use a class for global stuff
• prefer <STL container>.empty() over <STL container>.size() > 0
• make sure pluginDbg is efficient (noop when debug is not active)
• mumble_onAudioSourceFetched is too long (break down into helper functions; rel Make minimum noise/audio filters depending on radio model #122)
• use ‘const bool' locals to avoid complex if clauses

[hbeni]

make sure pluginDbg is efficient (noop when debug is not active)

checked that, precompiler statements excluide any code when non-debug, so the function is a noop.

[hbeni]

use ‘const bool' locals to avoid complex if clauses

Implemented already in current master, most of the part

[hbeni]

prefer .empty() over .size() > 0

Implemented in audio refactor branch

[hbeni]

mumble_onAudioSourceFetched is too long (break down into helper functions; rel Make minimum noise/audio filters depending on radio model #122)

I consider this fixed by #122; The function is still very long and hard to read. The latest PR to restructure the audio processing into the radio model helped already to make it easier/shorter and removed some of the obscure if-branches.

[hbeni]

• Use a class for global stuff

I skip this one on purpose. While I agree, it should be tackled some day, the more appropriate approach is surely the pending clean OO rewrite (#82 / #84).