Graphics: Change filename to source

[crigaut]

Fixes #6360
Changes parameter, property and attribute filename to source to be consistent with the image module: https://kivy.org/doc/stable/api-kivy.uix.image.html

[inclement]

Thanks, code changes look fine, there's just the issue of it being a breaking change. It sounds like we'll be okay with that for Kivy 2.0, but I'll leave it for someone more familiar with the svg code to merge.

[tshirtman]

I'm fine with this change, if needed it's easy to add an alias for compatibility, but this is an easy to fix update anyway, we just need to announce it as part of the breaking changes.