Refreshing SchemaSource every 3 seconds is clearly not the correct way to do things –it was just the best that could be done without adapting the GLib. So now that’s working, it would be great to add in the GLib the functions needed to do that a correct way, by monitoring changes in the schema dirs.

I’m bad at low-level abstractions, but that probably needs:
 * monitoring every directory with schemas (or empty) detected at first pass using g_file_monitor_directory;
 * monitoring the creation of the missing directories that might contains schemas (not sure how g_file_monitor_directory acts then);
 * maybe a function to say “start/stop monitoring”?
 * a signal for saying an update is needed;
 * and a g_settings_schema_source_get_updated function returning an updated SchemaSource, of course. ^^