GioFileTypeDetector always calls deprecated g_type_init

GioFileTypeDetector calls g_type_init, without checking whether dlsym successfully located it. g_type_init has been deprecated since Glib 2.36 as it is initialized automatically [1]. At some point in the future it will be removed which will cause problems for GioFileTypeDetector. Checks should be done to ensure the function exists before attempting to call it.

[1] https://developer.gnome.org/gobject/unstable/gobject-Type-Information.html#g-type-init