[Application] Add --run-as-service and ApplicationServiceProvider

When the switch '--run-as-service' is used, Crosswalk doesn't launch any
application, but only sits on its mainloop. The idea is that external
requests will happen to trigger app launches.

ApplicationServiceProvider will be the entity responsible for exposing
some interface for external requests. In Linux the current plan is to
expose a D-Bus service.

The Provider receives a pointer to ApplicationService, the idea behind
this design is that all the actual functionality should be implement in
ApplicationService, and the Provider will mostly bridge that
functionality through an external interface.
[Application] Add ApplicationServiceProvider for Linux

The ApplicationServiceProviderLinux will use a D-Bus service to expose
objects and let external environment manage applications as well as
launch applications.

For now a placeholder object that answers the message "Ping" with a
"Pong" response is there. Next commits will replace it with a real
object that exposes functionality from ApplicationService.