libdebugger: Implement remaining remote request/response pairs.

libdebugger: Fix header order in RemoteDebugRequest.

libdebugger: Initial work for remote support.

- Introduce base classes RemoteDebug{Request,Response} and the first set of
implementing subclasses for the various types of debug requests. These
encapsulate the information needed to make a request to the target debugger
interface, as well as a means to flatten/unflatten to a BMessage. These will
be used to marshal the relevant information for transmission across the
eventual transport interface. Not used anywhere yet since there still remain
some requests and responses to implement, among other things.