moon-view: Prefer g_auto* macros

g_autofree will automatically free variables when they go out of
scope. g_autoptr will do the same, but using the appropriate freefunc
for the given type, e.g. GDateTime -> g_date_time_unref() or
GObject -> g_object_unref().

This automatic cleanup reduces the risk of memory leaks
and allows getting rid of some lines of code.

src/window/moon-page/moon-view.c

@@ -139,13 +139,12 @@ picplanner_moon_set_illumination_intensity (PicplannerMoon *moon,
                                             double illumination,
                                             char *phase)
 {
-  char *char_label_illumination;
+  g_autofree char *char_label_illumination = NULL;
 
   char_label_illumination = g_strdup_printf ("%d%%", (int)round(illumination));
   gtk_label_set_text (GTK_LABEL (moon->label_illumination), char_label_illumination);
 
   gtk_label_set_text (GTK_LABEL (moon->label_phase), phase);
-  g_free (char_label_illumination);
 }