Make default_icon a path

Currently is always treated as a plain string so no expansion of `~`
happens. The real resolution (name of icon vs. path to icon) is done
later when the notification is about to be rendered.

src/settings_data.h

@@ -522,7 +522,7 @@ static const struct setting allowed_settings[] = {
                 .name = "default_icon",
                 .section = "*",
                 .description = "The default icon that is used when no icon is passed",
-                .type = TYPE_STRING,
+                .type = TYPE_PATH,
                 .default_value = "*",
                 .value = NULL,
                 .parser = NULL,