Add custom class init and instance init functions [glibmm-2-58]
Custom widget implementations should be able to contain code that is executed in
the class init function or the instance init function. E.g.
can only be called in the class init function, and
should only be called in the instance init function.
This is now possible in the newest ABI-breaking releases in the master branch,
at the moment with the ABI name
gtkmm-2.60. It would be fine to have this
possibility also without breaking the ABI of
This issue is also discussed in gtkmm#17 (closed).