stop using gtk_icon_info_free()

It's been deprecated.

https://bugzilla.gnome.org/show_bug.cgi?id=707538
stop using gtk_style_context_get_font

It's been deprecated.

https://bugzilla.gnome.org/show_bug.cgi?id=707538
bump GDK_VERSION_MIN_REQUIRED to 3.8

https://bugzilla.gnome.org/show_bug.cgi?id=707538