Introduce a new Persistable mixin

[falkoschindler]

This PR implements a Persistable mixin as an alternative to the existing persistence mechanism. It works as a standalone module without the need for a globally managed registry. To avoid cyclic imports, it doesn't even use RoSys' on_repeat and on_shutdown hooks, but uses NiceGUI's app.timer and app.on_shutdown instead.

A new example demonstrates the intended use:

• Classes can derive from Persistable if they want to support persistence. If so, they need to overwrite the abstract methods backup_to_dict() and restore_to_dict() to define the serialization and deserialization.
• The user can call persistent() to make individual objects persistent (builder pattern). This opt-in approach gives full control to the user and allows more customization (key, path, intervals, ...) than before.

At the moment I see the following remaining tasks:

• remove TODO markers in persistable.py (we keep the defaults)
• validate key (only letters, digits, dash, underscore)
• app.on_shutdown(self.sync_backup) --> or app.on_shutdown(self. _sync_backup_if_requested)
• remove delete_persistence()
• add pytests
• add documentation (how?)
• implement something like export_button and import_button for persistables?
• use new Persistable for builtin RoSys modules
• write migration guide: derive from Persistable, add .persistent() calls, rename backup and restore methods
• remove old persistence registry and PersistentModule class