gotk3_gtksource v0.9.11

(since 2021/03/27) current gotk3_gtksource version comply with new gotk3 requirements, (that mean gotk3 version must be > 0.5.2) so you have to use Go bindings for GTK3 @ master from gotk3 repository instead of Release GOTK3 Version 0.5.2.

Adding Go Modules support, which will be mandatory with the usage of go v1.17.