[Application][Linux] Separate the concept of a running application

This object represents to the D-Bus session bus the running application
object. It is moved from the Running Applications Manager to make it
easier to manage different running applications coming and going.

For now only the AppID property and the Terminate() method are
implemented.
[Application][Linux] Follow the service name of the launcher

When an application is launched via the xwalk-launcher tool, it makes
sense the application exits when the launcher tools exits. This is done
by following the D-Bus service name of the process that called the
'Launch()' method and killing the application when this name exits
the session bus.
[Application][Linux] Only allow the launcher to call Terminate

To avoid other processes to kill each other applications, we only allow
the process that called the 'Launch()' method to call 'Terminate()'
so each launcher cannot kill another's applications.
[Runtime] Clarify ApplicationService error messages