Document ConcurrentQueue and de-inline its implementation

ConcurrentQueue is currently used only by two classes and a test, but
modifying concurrent_queue.h requires recompiling 30 source files. None
of the member functions is so lightweight as to make it worth inlining.

An alternative to `@note ConcurrentQueue is unable to execute jobs if
@p threadCount == 0.` is `assert(threadCount != 0);`. But this would
force classes that contain a ConcurrentQueue data member to always start
a thread, even if they detect at runtime that they are never going to
enqueue a job.

Add Job type alias to avoid repeating the type.

Use default member initializers instead of the member initializer list
to make it clear [to the reader of the header] that no data member is
left uninitialized.

YACReaderLibrary/YACReaderLibrary.pro

@@ -160,6 +160,7 @@ HEADERS += comic_flow.h \
 }
 
 SOURCES += comic_flow.cpp \
+    ../common/concurrent_queue.cpp \
     create_library_dialog.cpp \
     db/comic_query_result_processor.cpp \
     db/folder_query_result_processor.cpp \