Moving grouping functionality to Mask/Group tab.

Re #6212.
Moved all grouping to Mask/Group tab. Re #6212.
Added shortcuts. Re #6212.
Improved pick image drawing. Re #6212.
Fix doxygen warning. Re #6212.
Merge remote-tracking branch 'origin/feature/6212_grouping_functions_in_mask_tab' Refs #6212

Conflicts:
	Code/Mantid/MantidPlot/src/Mantid/InstrumentWidget/InstrumentWindow.cpp