This file doesn’t contain any real implementation, it just call the
`impl` functions from the platform-specific files
`gcontenttype-{fdo,osx,win32}.[cm]`.

It serves as a location for the doc comments, introspection annotations
and API preconditions, and will be built on every platform. In
particular, this means that we get consistent GIR output for the
`g_content_type_*()` APIs regardless of whether GLib was built on Linux or
Windows or macOS.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3399