docs: fix up reference docs a bit

- require GTK-Doc 1.19
      - remove sgml mode
      - automatically generate gi.types (needs GTK-Doc 1.19)
      - fix https://bugzilla.gnome.org/show_bug.cgi?id=700025
[WIP] - rearange sections a bit
[WIP] - add gi-building, gi-programming sections
[WIP] - mark missing docs with TODO, which is only marginaly
        better than nothing but at least can be grepped :)

https://bugzilla.gnome.org/show_bug.cgi?id=571648

giinterfaceinfo.c

@@ -28,8 +28,8 @@
 
 /**
  * SECTION:giinterfaceinfo
- * @Short_description: Struct representing a GInterface
- * @Title: GIInterfaceInfo
+ * @title: GIInterfaceInfo
+ * @short_description: Struct representing a GInterface
  *
  * GIInterfaceInfo represents a #GInterface type.
  *
@@ -297,9 +297,10 @@ g_interface_info_get_signal (GIInterfaceInfo *info,
  * @info: a #GIInterfaceInfo
  * @name: Name of signal
  *
+ * TODO
+ *
  * Returns: (transfer full): Info for the signal with name @name in @info, or
  * %NULL on failure.
- *
  * Since: 1.34
  */
 GISignalInfo *