Re 6687 MantidEV menubar, clean up code

Removed print statements left over from when the methods
were just "stubs".  Fix variable scope style flagged by
CPP check.

Refs #6687
RE 6687 MantidEV menubar, clean up code -- 2

Added Doxygen comments to all methods.
Delete local thread pool in the destructor.

Refs #6687
Merge branch 'feature/6687_MantidEV_menubar' into develop