Use G_DEFINE_TYPE_WITH_PRIVATE if applicable

This gets rid of the strange `do_init` macro and makes the intent a bit
more clear.
5 jobs for wip/nielsdg/use-g-define-type-with-private in 51 minutes and 14 seconds (queued for 7 minutes and 27 seconds)
Status Job ID Name Coverage
  Preparation
passed gst indent #59819

00:00:21

passed manifest #59818

00:00:23

 
  Build
passed build fedora x86_64 #59820

00:04:42

 
  Full Builds
passed build android arm64 api28 #59821

00:04:19

passed build cerbero fedora x86_64 #59822

00:45:39