Make gobject_gen also work for stable rust (with a workaround)

gobject_gen required a nightly rust compiler to work, because of the procedural macro's.
It is possible to work around this by using a procedural macro. This macro
can contain an attribute which contains the gobject definition. The object where this
attributes is on is irrelevant and will not be used during derive.
2 jobs for stable_hack in 11 minutes and 47 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #97116
nightly:test

00:11:28

 
  Lint
passed #97117
rustfmt

00:00:19