Merge remote-tracking branch 'origin/feature/8281_sqw_interface_tidy'
Added a new icon for Remove Error Bars on the graph menu

To Test:
1. Start Mantidplot, load some data and create a line graph
1. With the graph selected open the graph menu
1. the Add error bars and remove error bars icons should be different and make sense.

re #3812
Merge branch 'feature/3812_Remove_error_bar_icon' into develop