milky-way-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/milky-way-page/milky-way-view.c

@@ -134,11 +134,10 @@ picplanner_milky_way_set_disturbance (PicplannerMilkyway 	*milky_way,
 																			double 							*coordinates_array_moon,
 																			double 							*coordinates_array_milky_way)
 {
-  char *char_label_disturbance_sun;
-  char *char_label_disturbance_moon;
-  char *char_label_no_disturbance;
-  char *char_label_possibly_no_disturbance;
-
+  g_autofree char *char_label_disturbance_sun = NULL;
+  g_autofree char *char_label_disturbance_moon = NULL;
+  g_autofree char *char_label_no_disturbance = NULL;
+  g_autofree char *char_label_possibly_no_disturbance = NULL;
 
   char_label_disturbance_sun = picplanner_get_char_disturbance (date_time_noon,
                                                                 -18, 360, -360,
@@ -171,11 +170,6 @@ picplanner_milky_way_set_disturbance (PicplannerMilkyway 	*milky_way,
                                                                         coordinates_array_milky_way);
 
   gtk_label_set_text (GTK_LABEL (milky_way->label_possible_disturbance_sun), char_label_possibly_no_disturbance);
-
-  g_free (char_label_disturbance_sun);
-  g_free (char_label_disturbance_moon);
-  g_free (char_label_no_disturbance);
-  g_free (char_label_possibly_no_disturbance);
 }