2.51.6:

This is version 2.51.6 of glibmm-2.24. Ignore versions 2.51.1 to 2.51.5 of
unstable glibmm-2.52. Unstable glibmm-2.52 is now unstable glibmm-2.54,
leaving the 2.51/52 version numbers again for use by stable glibmm-2.52.

Glib:
* Dispatcher:
  - autodeduce the type of the fd field.
  (Marcin Kolny) Bug #772074
  - Don't cast a HANDLE to an int on Windows.
  (Kjell Ahlstedt) Bug #772074
* ustring: Add cbegin() and cend().
  (Murray Cumming)

Gio:
* Action: Include variant.h in the heaer.
  (Daniel Boles) Bug #777953