Update GUI/operation_icons/absolute_grey.png,...
requested to merge 73-create-grey-icons-for-gui into 64-create-operation-box-with-buttons-in-the-gui
Update GUI/operation_icons/absolute_grey.png, GUI/operation_icons/addition_grey.png, GUI/operation_icons/butterfly_grey.png, GUI/operation_icons/complex_conjugate_grey.png, GUI/operation_icons/constant_grey.png, GUI/operation_icons/constant_multiplication_grey.png, GUI/operation_icons/custom_operation_grey.png, GUI/operation_icons/division_grey.png, GUI/operation_icons/input_grey.png, GUI/operation_icons/max_grey.png, GUI/operation_icons/min_grey.png, GUI/operation_icons/multiplication-grey.png, GUI/operation_icons/output_grey.png, GUI/operation_icons/register_grey.png, GUI/operation_icons/square_root_grey.png, GUI/operation_icons/subtraction_grey.png files
Closes #73 (closed)