Add Deposit and Withdraw Events to Balances Pallet

[apopiak]

addresses paritytech/polkadot#3508

[The missing events] make it difficult to trace transfers when these functions are called e.g. via XCM. (As surfaced by paritytech/polkadot#3489)

This PR adds Deposit, Withdraw and Slashed events as well as deposit_event calls where appropriate in order to make it easier to trace balance movements.

[shawntabrizi]

Can you add some justification why we need these events?

Seems to me we have been fine so far without them, and there is non-zero overhead of such things.

[apopiak]

Added a justification based on the linked Polkadot issue.
TLDR: Right now it's impossible to know funds were transfered via XCM (if you only follow events).

[xlc]

Even with this, from payment receiver point of view, is it possible to know where the fund is coming from?

[apopiak]

Even with this, from payment receiver point of view, is it possible to know where the fund is coming from?

Only if you track the fund movements on the source and target chains. As the receiver you would only see a Deposit which does not have a source.
We can explore having more events on the XCM side as well, that would need to be in Polkadot (and maybe Cumulus), though.

[xlc]

This is useful for exchanges to detect incoming deposits. But doesn't solve the problem of tracking funds. I guess we need some event system rework at some stage.

Still, we will want this get released ASAP otherwise it is impossible for exchanges to handling deposits via XCM.

[apopiak]

bot merge

[ghost]

Trying merge.

[TheGoldenEye]

In which runtime it will be active?

[shawntabrizi]

I think 0912 it should get in