Previously, some of the doc comments for platform-independent APIs were
in `gdesktopappinfo.c`, which is only built on Unix systems. This meant
the introspection annotations for those APIs were not used on non-Unix
systems, which caused platform differences in `Gio-2.0.gir`.

So, move those doc comments to `gappinfo.c` and put them next to some
new platform-independent wrapper functions which provide a consistent
entry point and location for the API preconditions.

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

Helps: #3399