getDeclarativeScriptEngine() is extracted into separate function

So, it is possible to get access to QScriptEngine just having access
to any declarative entity providing access to QDeclarativeContext

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jollamobile.com>
Error constructors are moved into module

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jollamobile.com>
Added abilty to send messages while processing request

Message processing function has now additional parameter provided by
Actor - object with .reply(data) method to send reply messages while
processing request. At the end of processing return result is also
sent to callback function.

Also performed huge refactoring - more consistent names etc.

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jollamobile.com>