esql_model: Implemented children_slice_get
esql_model: Added some Emodel functions I will probably use.
esql_model: Function should be static keyword.
esql_model: Few changes:

- using a single private header for both model and model_table (less files on disk);
- constructor->do(set ini data)[status unloaded] / emodel_load->do(create database connection instance and set connection callback function)[status loading] / connection_cb->do(set database name to use and dispatch query to list tables(children))[status loaded] / load_cb->do(list database tables, locally store emodel_table objects and dispatch EMODEL_EVENT_CHILDREN_COUNT_CHANGED).