Added new option to place and orient arrow source from the centre.

Arrow source tends to be placed a distance slightly off the centre of the entire arrow (depending on the length of its cylinder and tip) making scaling a bit off. Ideally having the arrow centralized in some cases makes it easier to achieve this. Standalone Unit-test for ArrowSource class. (changed const char* to std::string for ‘GetArrowOriginAsString’).

Backport: release