placesview: Set .error style if unsupported protocol

When the user types an address with a schema that is not supported,
the Connect button doesn't become sensitive, but there is no visible
feedback at all.

This feels unresponsive and leaves the user clueless.

While it doesn't help explain why the address doesn't work, this will
provide a hint that the input was acknowledged but doesn't work.

https://gitlab.gnome.org/GNOME/gtk/issues/1476

gtk/gtkplacesview.c

@@ -1809,6 +1809,13 @@ on_address_entry_text_changed (GtkPlacesView *view)
 
 out:
   gtk_widget_set_sensitive (priv->connect_button, supported);
+  if (scheme && !supported)
+    gtk_style_context_add_class (gtk_widget_get_style_context (priv->address_entry),
+                                 GTK_STYLE_CLASS_ERROR);
+  else
+    gtk_style_context_remove_class (gtk_widget_get_style_context (priv->address_entry),
+                                    GTK_STYLE_CLASS_ERROR);
+
   g_free (address);
   g_free (scheme);
 }