Added a comment on the documantation. Re #6160.
Put the buttons into groups. Re #6160.
Merge branch 'master' into feature/6160_mask_tab. Re #6160.

Conflicts:
	Code/Mantid/MantidPlot/src/Mantid/InstrumentWidget/InstrumentWindowMaskTab.cpp
Enable/disable buttons based on the state of mask. Re #6160
Merge branch 'master' into feature/6160_mask_tab. Re #6160.

Conflicts:
	Code/Mantid/MantidPlot/src/Mantid/InstrumentWidget/ProjectionSurface.h
Catch and ignore exception when masking invalid detector. Re #6160.
Merge remote-tracking branch 'origin/feature/6160_mask_tab'