Eliminate G_CONST_RETURN

The G_CONST_RETURN define in GLib is, and has always been, a bit fuzzy.

We always used it to conform to the platform, at least for public-facing
API.

At first I assumed it has something to do with brain-damaged compilers
or with weird platforms where const was not really supported; sadly,
it's something much, much worse: it's a define that can be toggled at
compile-time to remove const from the signature of public API. This is a
truly terrifying feature that I assume was added in the past century,
and whose inception clearly had something to do with massive doses of
absynthe and opium — because any other explanation would make the
existence of such a feature even worse than assuming drugs had anything
to do with it.

Anyway, and pleasing the gods, this dubious feature is being
removed/deprecated in GLib; see bug:

  https://bugzilla.gnome.org/show_bug.cgi?id=644611

Before deprecation, though, we should just remove its usage from the
whole API. We should especially remove its usage from Cally's internals,
since there it never made sense in the first place.