[stable24] Trigger event on content change in syncservice

[mejo-]

Manual backport of #2465

• Resolves: Trigger event on content change in syncService #2462
• Target version: master

Summary

Fires 4 events from SyncService

• sync-service:idle
• sync-service:save
• sync-service:error
• sync-service:sync

Prevention of Memory Leak

All SyncService event handlers are storage in arrays, and never removed from these arrays.
Although the SyncService whore removed from Vue component context, we had a circular reference here.
This is a very common scenario for Memory Leak in JavaScript.

Now using mitt library is possible to use .off to remove an event handler.

To do it, I've change part of event contracts (error event) and I've split the event handlers in component methods.

There are more potential problems to solve, but we can do it later.

[max-nextcloud]

yes... i guess would be good to bring this back to stable24