app: fix propgui random seed generation

After the switch of random-seed properties from INT to UINT, their
upper bound results in a negative value when converted to a
gint32, causing a CRITICAL in the call to g_random_int_range().
Use g_random_double_range() instead, which has enough precision to
accurately represent all values in the range, and round the result.

app/propgui/gimppropgui-utils.c

@@ -23,6 +23,7 @@
 #include <gegl.h>
 #include <gtk/gtk.h>
 
+#include "libgimpmath/gimpmath.h"
 #include "libgimpwidgets/gimpwidgets.h"
 
 #include "propgui-types.h"
@@ -211,8 +212,10 @@ static void
 gimp_prop_random_seed_new_clicked (GtkButton     *button,
                                    GtkAdjustment *adj)
 {
-  guint32 value = g_random_int_range (gtk_adjustment_get_lower (adj),
-                                      gtk_adjustment_get_upper (adj));
+  guint32 value;
+
+  value = floor (g_random_double_range (gtk_adjustment_get_lower (adj),
+                                        gtk_adjustment_get_upper (adj) + 1.0));
 
   gtk_adjustment_set_value (adj, value);
 }