dialogs: fix about dialog's "draw" signal handler

According to the GTK docs, the handler is supposed to have a gboolean
return type and return FALSE to propagate the event further.  With the
previous function signature, on some setups (such as Fedora 31 and 32)
the about dialog would show up as a black rectangle or be invisible.

Change-Id: I3d0d343df58b08f8cc2e7a9f97710dacc63f1c41

src/dialogs.cpp

@@ -41,11 +41,14 @@ about_dialog_set_theme(GtkAboutDialog *dialog, gboolean use_dark_theme)
     gtk_about_dialog_set_logo(dialog, logo);
 }
 
-void
-about_dialog_on_redraw(GtkWidget *dialog)
+gboolean
+about_dialog_on_redraw(GtkWidget *dialog,
+                       G_GNUC_UNUSED cairo_t *cr,
+                       G_GNUC_UNUSED gpointer user_data)
 {
-  about_dialog_set_theme(GTK_ABOUT_DIALOG(dialog),
-                         use_dark_theme(get_ambient_color(dialog)));
+    about_dialog_set_theme(GTK_ABOUT_DIALOG(dialog),
+                           use_dark_theme(get_ambient_color(dialog)));
+    return FALSE;
 }
 
 void