vcl: Remove DummyApplication

DummyApplication is, frankly, a hack. Application is an abstract class because
Application::Main() is a pure virtual function. However, as DummyApplication
shows - this is not necessary because InitVCL() just makes a dummy app anyway.
Anyone who uses Application::Main() will get a warning.

Also: I want to move some of the global data into Application, and I can't do this
whilst it is an abstract class. Given it's not necessary, then I'm getting rid of
Main() as a pure virtual function.

Change-Id: I425cf6feae4866f5670963ee9905f6161e421dd2