Fixed a GafferImage bug involving filters

It turns out that when using Gaffer in maya, calling GafferImage.FilterPlug.filters() reinitializes the static string vector defined in GafferImage::FilterPlug::filters(), and wipes out all the filters, leading to a load of errors when you try and use nodes like the Reformat. This seems to be because it was defined in the header, and therefore defined separately in a number of .cpp files, including GafferImage/Filter.cpp, GafferImageBindings/FilterPlugBinding.cpp and GafferImageBindings/FilterBinding.cpp. How and why this works in standalone gaffer but not maya is a mystery to me, but defining these functions in Filter.cpp fixes the problem, and is also a bit more consistent with our style, so this is what I've done.