g_settings_bind: use canonical property name

g_settings_bind: use canonical property name

We were using the user-passed value of the @property argument for
several purposes in g_settings_bind(): error messages, binding
uniqueness (ie: one-binding-per-property-per-object) and most
importantly, connecting to the detailed notify:: signal.

The user may pass a string like "property_name" when the property's
canonical name is "property-name".  g_object_class_find_property() will
find the property under these circumstances, but a connection to
"notify::property_name" will not notice notifies emitted for
"property-name".

We can solve this by using the user's string to perform the lookup and
then using pspec->name for everything after that.

https://bugzilla.gnome.org/show_bug.cgi?id=684882