core: always use gulong to store signal handler ids

core: always use gulong to store signal handler ids

We inconsistently use gulong,guint,int types to store signal handler
id, but the type returned by g_signal_connect() is a gulong.

This has no practical consequences because a int/guint is enough to
store the value, however it is better to use a consistent type, also
because nm_clear_g_signal_handler() accepts a pointer to the signal id
and thus it must be always called with the same pointer type.