Implement the main form of operations view

This is a initial implementation of the form corresponding to the operations
view. It provides the controls necessary to insert the properties of a exchange
and submit it to the broker set.
Several improvements on UI order submission form

* The price model was inverted. The limit order should be enabled and the
market-price order should be disabled. It was the other way around, what has
been fixed.
* Use a custom `DecimalNumberTextField` that only accepts a decimal number
as input. This custom control implements the necessary event filters and
property listeners to forbid inputs that doesn't match a double number. This
new control is used for all text fields where a decimal number is expected.
* Inject a function to process the form submit. By default, this only shows
a dialog informing the order was submitted.
* Remove innecesary fields for custom priced orders.
* Require a valid bitcoin amount to enable the _submit_ button.
* Adecuate some labels to finantial jargon
* Some style issues fixed.
Merge pull request #216 from apoloval/issues/80

Implement the main form of operations view
Add more GUI tests
Small refactor in OrderSubmissionActor
Merge pull request #223 from edrevo/small-refactor

Small refactor in OrderSubmissionActor
Merge pull request #221 from edrevo/add-ui-tests

Add more GUI tests
Consider Order class as a domain model type

The order class was placed in `protocol.messages.brokerage` package, since it
was considered as part of a protocol message. But this type is actually
representing an entity of the Coinffeine domain model. Thus, the class has been
moved to the `common` package until we have a better place to store our
domain model classes.
Introduce an interface to the broker in Coinffeine API

The `CoinffeineApp` trait, which provides the API of a Coinffeine peer,
lacks a way to interact with the broker. This change provides such an interface
and its default implementation, which uses the `PeerActor` instance to
request the submission of orders.
Merge pull request #218 from apoloval/issues/82

Integration of UI and order submission logic