glib: add auto_cleanup functions so that consumers can use g_auto*

This is of great help as consumers of the GLib API. This way we can define things like

g_autoptr (PopplerSignatureInfo) info = NULL;

And it will get auto-freed at the end of the current scope

@nbenitez @mkasik @chpe as people usually pinged for glib things. I want to use g_auto macros in Papers 😄