Add Glib::VariantDict, needed for GApplication.

* glib/src/filelist.am:
* glib/src/variantdict.[hg|ccg]: Add it as a generic refcounted
  type, with templated lookup_value() and insert_value() methods,
  though these are completely untested so far.
* glib/src/variant.hg: Ignore a g_variant_dict_* function that
  gmmproc thinks is part of Variant.
* tools/m4/convert_gio.m4:
* tools/m4/convert_glib.m4: Move all Variant* conversion to the
  glib conversions and add a necessary one.
* glib/glibmm.h: Include variantdict.h.
Add a TODO comment