Rename fields of `Connection{,s}`
Rename fields of `Connection{,s}`

FBX SDK document [FBX 2017 Developer Help:
Connections](http://help.autodesk.com/view/FBX/2017/ENU/?guid=__files_GUID_BB63A93A_7663_4256_B060_8EA35CB0FF3A_htm)
uses the words "source" and "destination", and I decided to use these
words in the fbxcel code.

l1048576/fbxcel