Merge remote branch 'origin/feature/6845_export_log'
Re #6846. Moved WindowHelper to be in MantidQt/API
Re #6846. Added namespace to HelpWindow.
Re #6846. Changed HelpWindow to be a singleton.
Re #6846. Moved the algorithm help to use qt assistant.
Merge branch 'feature/6846_qtassistant_singleton' into develop