Execute catalog algorithms asynchronously. Refs #8321.

- Added a helper method to execute asynchronously to prevent duplication of code.
Make loading asynchronous. Refs #8321.
Const correctness. Refs #8321.
Fixed ICAT doxygen warnings. Refs #8321.
Merge remote-tracking branch 'origin/feature/8321_icat_asynchronous_algorithms'

Conflicts:
	Code/Mantid/MantidQt/MantidWidgets/inc/MantidQtMantidWidgets/ICatSearch2.h
Refs #8321