shared: add nm_g_{idle,timeout}_add_source() helpers

We have g_idle_add() and g_timeout_add(). But these return
those odd guint source ids. That is totally pointless. The
only potential benefit is that a guint is only 4 bytes while
a pointer is 8 bytes (on 64 bit systems). Otherwise, it seems
always preferable to have an actual GSource instance instead
of an integer. It also saves the overhead in g_source_remove()
which first needs to do a hash lookup to find the GSource.
A GSource instance would theoretically work with multiple
GMainContext instances, while g_source_remove() only works
wit g_main_context_default().

On the other hand we have helper API like nm_g_idle_source_new()
and nm_g_timeout_source_new(), which is fully flexible and sensible
because it returns a reference to the GSource instance. However, it
is a bit verbose to use in the common case.

Add helper functions that simplify the use and are conceptionally
similar to g_{idle,timeout}_add() (hence the name).

shared/nm-glib-aux/nm-shared-utils.h

@@ -1364,6 +1364,59 @@ nm_g_source_attach(GSource *source, GMainContext *context)
     return source;
 }
 
+static inline GSource *
+nm_g_idle_add_source(GSourceFunc func, gpointer user_data)
+{
+    /* G convenience function to attach a new timeout source to the default GMainContext.
+     * In that sense it's very similar to g_idle_add() except that it returns a
+     * reference to the new source.  */
+    return nm_g_source_attach(nm_g_idle_source_new(G_PRIORITY_DEFAULT, func, user_data, NULL),
+                              NULL);
+}
+
+static inline GSource *
+nm_g_timeout_add_source(guint timeout_msec, GSourceFunc func, gpointer user_data)
+{
+    /* G convenience function to attach a new timeout source to the default GMainContext.
+     * In that sense it's very similar to g_timeout_add() except that it returns a
+     * reference to the new source.  */
+    return nm_g_source_attach(
+        nm_g_timeout_source_new(timeout_msec, G_PRIORITY_DEFAULT, func, user_data, NULL),
+        NULL);
+}
+
+static inline GSource *
+nm_g_timeout_add_source_seconds(guint timeout_sec, GSourceFunc func, gpointer user_data)
+{
+    /* G convenience function to attach a new timeout source to the default GMainContext.
+     * In that sense it's very similar to g_timeout_add_seconds() except that it returns a
+     * reference to the new source.  */
+    return nm_g_source_attach(
+        nm_g_timeout_source_new_seconds(timeout_sec, G_PRIORITY_DEFAULT, func, user_data, NULL),
+        NULL);
+}
+
+static inline GSource *
+nm_g_timeout_add_source_approx(guint       timeout_msec,
+                               guint       timeout_sec_threshold,
+                               GSourceFunc func,
+                               gpointer    user_data)
+{
+    GSource *source;
+
+    /* If timeout_msec is larger or equal than a threshold, then we use g_timeout_source_new_seconds()
+     * instead. */
+    if (timeout_msec / 1000u >= timeout_sec_threshold)
+        source = nm_g_timeout_source_new_seconds(timeout_msec / 1000u,
+                                                 G_PRIORITY_DEFAULT,
+                                                 func,
+                                                 user_data,
+                                                 NULL);
+    else
+        source = nm_g_timeout_source_new(timeout_msec, G_PRIORITY_DEFAULT, func, user_data, NULL);
+    return nm_g_source_attach(source, NULL);
+}
+
 NM_AUTO_DEFINE_FCN0(GMainContext *, _nm_auto_unref_gmaincontext, g_main_context_unref);
 #define nm_auto_unref_gmaincontext nm_auto(_nm_auto_unref_gmaincontext)