Add string identifiers to Ether & ERC20 dividends

[adamdossa]

We would need to have a name / title field on the dividends struct so they are better identifiable.

struct Dividend {
  uint256 checkpointId;
  string name;
  ...
}

When a dividend is created it should also require a name to be passed and event should emit that as well.

Relates to the EtherDividendCheckpoint and ERC20DividendCheckpoint modules.