idb_sys: expose the Database from the VersionChangeEvent

[Ekleog]

Hey! Considering #7, I'm trying to use idb-sys for my database handling.

I'm just curious, is there any reason why this code is in idb, and not exposed straight from the VersionChangeEvent of idb-sys?

idb/idb/src/database.rs

Lines 129 to 132 in 4576249

	fn get_database_from_event(event: Event) -> Result<Database, Error> {
	let target = event.target().ok_or(Error::EventTargetNotFound)?;
	Database::try_from(target).map_err(Into::into)
	}

Anyway, it looks like idb-sys will be a great way for me to have both the reliability I'm looking for, but also some relative ease-of-use compared to web-sys and its untyped API. So, thank you! :D

[Ekleog]

Incidentally, it'd be great if the on_success / on_error callbacks could return an idb_sys::Event type, that'd similarly provide for easy access to the underlying database :)

[devashishdxt]

Fixed in #19.