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.
Up to now, the "include" directory contained (only) header files that were
used project-wide by libs, core, clients, et al.
Since the directory now also contains a non-header file, the "include"
name is misleading. Instead of adding yet another directory that is
project-wide, with non-header-only content, rename the "include"
directory to "shared".
