Now that g_object_ref() propagates the parameter type to the
return value, we need to cast to ensure the result is warning
free.

https://bugzilla.gnome.org/show_bug.cgi?id=790697

Sub-directories inside gio/ already processed in a previous commit:
- fam/
- gdbus-2.0/ (which contains only codegen/)
- gvdb/
- inotify/
- tests/
- win32/
- xdgmime/

Other sub-directories inside gio/:
- completion/: no license headers
- kqueue/: not LGPL, BSD-style license

https://bugzilla.gnome.org/show_bug.cgi?id=776504

If we have an input parameter (or return value) we need to use (nullable).
However, if it is an (inout) or (out) parameter, (optional) is sufficient.

It looks like (nullable) could be used for everything according to the
Annotation documentation, but (optional) is more specific.

Previously this would cause an assertion failure when checking the paths
of exported objects, as it would try to check that their paths started
with ‘//’ due to mishandling the root object case.

Includes a unit test.

https://bugzilla.gnome.org/show_bug.cgi?id=761810

Otherwise people might try to export the object manager at ‘/’, which
doesn’t work. And I have no intention of making it work.

https://bugzilla.gnome.org/show_bug.cgi?id=760852

The previous commit introduced a possible memory leak in cases
where we get a G_IO_ERROR_CLOSED error. Make sure to always
free an error, if we got one.

https://bugzilla.gnome.org/show_bug.cgi?id=753278

There are real world cases where emitting signals can fail, such
as if the DBus connection closes. Asserting and aborting the process
in these cases is just plain lazy.

Ignore the errors when the connection is closed, and turn the
others into warnings.

https://bugzilla.gnome.org/show_bug.cgi?id=753278

Strip lots of entity use from |[ ]| examples (which are now
implicit CDATA). Also remove many redundant uses of <!-- -->.