ZmqLogger: Add optional dumping to stderr

[ferdnyc]

Add an openshot::Settings boolean DEBUG_TO_STDERR, which when set true will direct all AppendDebugMethod() messages to std::clog, in addition to (possibly) being sent over the ZeroMQ wire.

Set to WIP because I want to add unit tests and check it over a bit more, right now this is a quick and dirty proof-of-concept implementation intended for #420.

Also:

• Replaced usleep() call with the C++11 std::this_thread::sleep_for()
• Added std:: prefixes throughout ZmqLogger.cpp, and removed the using namespace std;
• Corrected some cut-and-paste comment errors in Settings.h
• Reformatted a bunch of code for readability / differing tab widths

[codecov-io]

Codecov Report

Merging #422 into develop will decrease coverage by 0.00%.
The diff coverage is 36.36%.

@@             Coverage Diff             @@
##           develop     #422      +/-   ##
===========================================
- Coverage    49.45%   49.44%   -0.01%     
===========================================
  Files          129      129              
  Lines        10261    10265       +4     
===========================================
+ Hits          5075     5076       +1     
- Misses        5186     5189       +3     

Impacted Files	Coverage Δ
src/Settings.h	50.00% <ø> (ø)
src/ZmqLogger.cpp	28.20% <22.22%> (-1.13%)	⬇️
src/Settings.cpp	100.00% <100.00%> (ø)
src/ZmqLogger.h	33.33% <100.00%> (ø)

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[jonoomph]

@ferdnyc Is this PR ready for merge? It seems complete to me, but it still has the WIP decorator. If it's ready, slam that merge button! 👍 Thx.

[ferdnyc]

@jonoomph

I think it is ready. I had originally wanted to add unit tests, but I've since reconsidered given the issues I've already encountered trying to unit-test singletons. (Plus, I think we'd need a more full-stack unit test framework than just UnitTest++, to test output to stderr in any meaningful way.)

I'll see what state it's in and finish up whatever's needed, it would be good to have. I also wanted to integrate the functionality into OpenShot somehow, and with --debug support merged things are in a much more conducive state over on that side.

[jonoomph]

Thanks, LGTM!