Add `Balances` config variable to `prediction-markets` and use a variant of `orml_tokens::CurrencyAdapter` to abstract `Balances`

[maltekliemann]

In some situations, we require direct access to the Balances pallet instead of going through MultiCurrency. Using imbalances would be one example.

We're currently using CurrencyOf<T>, which "imports" the Balances pallet from MarketCommons. That's one way to abstract the Balances pallet.

Th state-of-the-art solution appears to be to just add a config variable to every pallet that requires direct access. This can be "prettified" by implementing something similar to orml_tokens::CurrencyAdapter for orml_currencies and the using CurrencyAdapter<Currencies, GetNativeToken> instead of Balances.

After this is done, we should also be able to eliminate the Currency config variable from MarketCommons.

[maltekliemann]

I'm not sure if this is worth the hassle.

[maltekliemann]

Probably outdated.