shortcutswindow: Reduce default width of search entry

Reduce the default width of search entry so that it fits on smaller
screens (ie, screens having 360px or less width).  Also, set max width
to the old value of 40, so that the search entry will have the same
old size if window width permits.

This commit won't make any difference on larger screens.

gtk/gtkshortcutswindow.c

@@ -944,7 +944,8 @@ gtk_shortcuts_window_init (GtkShortcutsWindow *self)
   g_object_set (self->search_entry,
                 /* Translators: This is placeholder text for the search entry in the shortcuts window */
                 "placeholder-text", _("Search Shortcuts"),
-                "width-chars", 40,
+                "width-chars", 31,
+                "max-width-chars", 40,
                 NULL);
 
   gtk_accessible_update_property (GTK_ACCESSIBLE (self->search_entry),