Silence some gir warnings by hiding stuff

* Hide gnt_text_format_flag_to_chtype.
It's not used downstream in Finch, and is an implementation detail.

* Make gnt_widget_get_window return a gpointer.
Returning a `WINDOW *` causes gir to complain about unknown types. I'd rather
just remove this function, but it appears to be used by the window manager
plugins.

Testing Done:
Compile tested only.

Reviewed at https://reviews.imfreedom.org/r/55/