Add the GLIB_AVAILABLE_IN_ALL annotation to all old functions (that
haven't already been annotated with the GLIB_AVAILABLE_IN_* macros or a
deprecation macro).
If we discover in the future that we cannot use only one macro on
Windows, it will be an easy sed patch to fix that.
https://bugzilla.gnome.org/show_bug.cgi?id=688681
Move g_pollable_source_new() here from gpollableinputstream.c, add
g_pollable_source_new_full(), and add some new methods to do either
blocking or nonblocking reads depending on a boolean argument.
https://bugzilla.gnome.org/show_bug.cgi?id=673997