Use exchange id as blocked fiat funds identifier

This moves the responsibility of generating identifiers from the
payment processor to their clients. Remove BlockedFundsId and use
ExchangeId in return.
Remove redundant BlockedFunds.fiat field

As fiat funds id is the exchange id this field has become redundant.
Merge pull request #306 from apoloval/task/cf-100

Add infrastructure to manage app preferences
Notify fiat funds availability on the event stream
Rename BlockingFundsActor to BlockedFiatRegistry

The name of the class was easy to be misunderstood as the one asking
for the funds.
Remove listeners from BlockedFiatRegistry
Merge pull request #307 from sortega/issue/108_persist_fiat_funds

Refactor fiat funds blocking
Make settings provider available from preferences form

And use it to fill the current settings values in the form.
Make OK Pay preferences tab to save its config on close

Now we finally have a mechanism to save the OKPay settings.
Merge pull request #308 from apoloval/task/cf-100

Make preferences form to save the settings