Add G_VALUE_INIT

Add G_VALUE_INIT

The implementation of GValue is not public or documented.  When
allocated on the stack, initializing a GValue is usually done as
documented with:

GValue value = { 0, };

There is lot code around (including WebKit) that added all the missing
fields, resulting in this ugly and non-obvious:

GValue value = { 0, { { 0 } } };

However, this doesn't play nice with -Wmissing-field-initializers for
example. Thus, G_VALUE_INIT.

http://bugzilla.gnome.org/show_bug.cgi?id=654793
http://bugzilla.gnome.org/show_bug.cgi?id=577231