[Application] Add dbus agent to talk with runtime and launcher

The DBus agent will be created for DBus communication between xwalk runtime and
launcher. The instance of this class will be owned by Application class.

Implementation details:
The runtime app proxy will be created from runtime's running application
interface, and provides hide & terminiate functions. The launcher app proxy will
be created from launcher's interface, and provides launch & kill functions.
[Application] Add ApplicationManager to encapsulate application related info and operations

The ApplicationManager will be created by ApplicationExtension to encapsulate basic
application information and operations.
[Application] Add ApplicationContext class to provide app context related information

The application context ID is a running Tizen app's PID which can be retrieved
from AMD(Application Management Daemon implemented in AUL package). We need to
fetch such info from xwalk-launcher.
[Application] Add application context and app launch/kill support

The supported APIs are listed below:
  ApplicationManager.getCurrentApplication(),
  ApplicationManager.kill(),
  ApplicationManager.launch(),
  and Application, ApplicationContext intefaces.

The application launch/kill operations will be implemented in xawlk-launcher and
triggered by DBus client in the DBus agent.