Add an asynchronous version of gdk_pixbuf_get_file_info

https://bugzilla.gnome.org/show_bug.cgi?id=712599
Add gdk_pixbuf_read_pixels and gdk_pixbuf_new_from_bytes to the docs

https://bugzilla.gnome.org/show_bug.cgi?id=735410
 Updates for 2.31.1
Bump version