Replace manual enum functions with autogenerated ones

See https://github.com/gtk-rs/gir/pull/991: gir learned to generate associated functions for enums: this MR contains partially regenerated files with those functions and deletes manual implementations, while at the same time reworking some of the intermediate impl blocks. There are still some issues to be resolved which will be created as new threads below instead of a bulletpoint list that's annoying to respond to :)

Note that, for the time being, this needs an MSRV of 1.42 - which is apparently planned?