GdkContentFormatsBuilder is currently not introspectable, as it does not
have a GType. We can turn it into a boxed type, but we need to implement
memory management for it.

The current gdk_content_formats_builder_free() function returns a newly
constructed value, so we cannot use it as a GBoxedFreeFunc; additionally
copying a GdkContentFormatsBuilder contents would make it a bit odd, as
you could get multiple identical GdkContentFormats out of the copies.

A simple approach is to model the GdkContentFormatsBuilder API to follow
the GBytes one: use reference counting for memory management, and have
a function to release a reference, return a GdkContentFormats, and reset
the GdkContentFormatsBuilder state.

For language bindings, we can provide a get_formats() function that
returns the GdkContentFormats instance and resets the builder instance,
leaving the reference count untouched.

For C convenience we can keep gdk_content_formats_builder_free(), and
make it a wrappe...